[11]This acid character is explained by the influence of the platinum on the hydrogen, and by the attachment of the cyanogen groups. Thus cyanuric acid, H3(CN)3O3, is an energetic acid compared with cyanic acid, HCNO. And the formation of a compound with five molecules of water of crystallisation, (PtH2(CN)4,5H2O), confirms the opinion that platinum is able to form compounds of still higher types than that expressed in its saline compounds, and, moreover, the combination of hydroplatinocyanic acid with water does not reach the limit of the compounds which appears in PtCl4,2HCl,6H2O.A whole series ofplatinocyanidesof the common type PtR2(CN)4nH2O is obtained by means of double decomposition with the potassium or hydrogen or silver salts. For example, the salts of sodium and lithium contain, like the potassium salt, three molecules of water. The sodium salt is soluble in water and alcohol. The ammonium salt has the composition Pt(NH4)2(CN)4,2H2O and gives crystals which reflect blue and rose-coloured light. This ammonium salt decomposes at 300°, with evolution of water and ammonium cyanide, leaving a greenishplatinum dicyanide, Pt(CN)2, which is insoluble in water and acid but dissolves in potassium cyanide, hydrocyanic acid, and other cyanides. The same platinous cyanide is obtained by the action of sulphuric acid on the potassium salts in the form of a reddish-brown amorphous precipitate. The most characteristic of the platinocyanides are those of the alkaline earths. The magnesium salt PtMg(CN)4,7H2O crystallises in regular prisms, whose side faces are of a metallic green colour and terminal planes dark blue. It shows a carmine-red colour along the main axis, and dark red along the lateral axes; it easily loses water, (2H2O), at 40°, and then turns blue (it then contains 5H2O, which is frequently the case with the platinocyanides). Its aqueous solution is colourless, and an alcoholic solution deposits yellow crystals. The remainder of the water is given off at 230°. It is obtained by saturating platinocyanic acid with magnesia, or else by double decomposition between the barium salt and magnesium sulphate. The strontium salt, SrPt(CN)4,4H2O crystallises in milk-white plates having a violet and green iridescence. When it effloresces in a desiccator, its surfaces have a violet and metallic green iridescence. A colourless solution of the barium salt PtBa(CN)4,4H2O is obtained by saturating a solution of hydroplatinocyanic acid with baryta, or by boiling the insoluble copper platinocyanide in baryta water. It crystallises in monoclinic prisms of a yellow colour, with blue and green iridescence; it loses half its water at 100°, and the whole at 150°. The ethyl salt, Pt(C2H5)2(CN)4,2H20, is also very characteristic; its crystals are isomorphous with those of the potassium salt, and are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatinocyanic acid. The facility with which they crystallise, the regularity of their forms, and their remarkable play of colours, renders the preparation of the platinocyanides one of the most attractive lessons of the laboratory.By the action of chlorine or dilute nitric acid, the platinocyanides are converted into salts of the composition PtM2(CN)5, which corresponds with Pt(CN)3,2KCN—that is, they express the type of a non-existent form of oxidation of platinum, PtX3(i.e.oxide Pt2O3), just as potassium ferricyanide (FeCy3,3KCy) corresponds with ferric oxide, and the ferrocyanide corresponds with the ferrous oxide. The potassium salt of this series contains PtK2(CN)5,3H2O, and forms brown regular prisms with a metallic lustre, and is soluble in water but insoluble in alcohol. Alkalis re-convert this compound into the ordinary platinocyanide K2Pt(CN)4, taking up the excess of cyanogen. It is remarkable that the salts of the type PtM2Cy5contain the same amount of water of crystallisation as those of the type PtM2Cy4. Thus the salts of potassium and lithium contain three, and the salt of magnesium seven, molecules of water, like the corresponding salts of the type of platinous oxide. Moreover, neither platinum nor any of its associates gives any cyanogen compound corresponding with the oxide,i.e.having the composition PtK2Cy6, just as there are no compounds higher than those which correspond to RCy3nMCy3for cobalt or iron. This would appear to indicate the absence of any such cyanides, and indeed, for no element are there yet known any poly-cyanides containing more than three equivalents of cyanogen for one equivalent of the element. The phenomenon is perhaps connected with the faculty of cyanogen of giving tricyanogen polymerides, such as cyanuric acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution of PtK2(CN)4gives (besides PtK2Cy5) a product PtK2Cy4Cl2, which evidently contains the form PtX4, but at first the action of the chlorine (or the electrolysis of, or addition of dilute peroxide of hydrogen to, a solution of PtK2Cy4, acidulated with hydrochloric acid) produces an easily soluble intermediate salt which crystallises in thin copper-red needles (Wilm, Hadow, 1889). It only contains a small amount of chlorine, and apparently corresponds to a compound 5PtK2Cy4+ PtK2Cy4Cl2+ 24H2O. Under the action of an excess of ammonia both these chlorine products are converted either completely or in part (according to Wilm ammonia does not act upon PtK2Cy4) into PtCy2,2NH3,i.e.a platino-ammonia compound (seefurther on). It is also necessary to pay attention to the fact that ruthenium and osmium—which, as we know, give higher forms of oxidation than platinum—are also able to combine with a larger proportion of potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a crystallinehydroruthenocyanic acid, RuH4(CN)6, which is soluble in water and alcohol, and corresponds with the salts M4Ru(CN)6. There are exactly similar osmic compounds—for example, K4Os(CN)6,3H2O. The latter is obtained in the form of colourless, sparingly-soluble regular tablets on evaporating the solution obtained from a fused mixture of potassium osmiochloride, K2OsCl6, and potassium cyanide. These osmic and ruthenic compounds fully correspond with potassium ferrocyanide, K4Fe(CN)6,3H2O, not only in their composition but also in their crystalline form and reactions, which again demonstrates the close analogy between iron, ruthenium, and osmium, which we have shown by giving these three elements a similar position (in the eighth group) in the periodic system. For rhodium and iridium only salts of the same type as the ferricyanides, M3RCy6, are known, and for palladium only of the type M2PdCy4, which are analogous to the platinum salts. In all these examples aconstancy of the typesof the double cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, gold. The double cyanides of iron, ruthenium, osmium have the type K4R(CN)6; of cobalt, rhodium, iridium, the type K3R(CN)6; of nickel, palladium, platinum the type K2R(CN)4and K2R(CN)5; and for copper, silver, gold there are known KR(CN)2, so that the presence of 4, 3, 2, and 1 atoms of potassium corresponds with the order of the elements in the periodic system. Those types which we have seen in the ferrocyanides and ferricyanides of iron repeat themselves in all the platinoid metals, and this naturally leads to the conclusion that the formation of similar so-called double salts is of exactly the same nature as that of the ordinary salts. If, in expressing the union of the elements in the oxygen salts, the existence of anaqueous residue(hydroxyl group) be admitted, in which the hydrogen is replaced by a metal, we have then only to apply this mode of expression to the double salts and the analogy will be obvious, if only we remember that Cl2, (CN)2, SO4, &c., are equivalent to O, as we see in RO, RCl2, RSO4, &c. They all = X2, and, therefore, in point of fact, wherever X (= Cl or OH, &c.) can be placed, there (Cl2H), (SO4H), &c., can also stand. And as Cl2H = Cl + HCl and SO4H = OH + SO3, &c., it follows that molecules HCl or SO3, or, in general, whole molecules—for instance, NH3, H2O, salts, &c., can annex themselves to a compound containing X. (This is an indirect consequence of the law of substitution which explains the origin of double salts, ammonia compounds, compounds with water of crystallisation, &c., by one general method.) Thus the double salt MgSO4,K2SO4, according to this reasoning,may beconsidered as a substance of the same type as MgCl2, namely, as = Mg(SO4K)2, and the alums as derived from Al(OH)(SO4), namely, as Al(SO4K)(SO4). Without stopping to pursue this digression further, we will apply these considerations to the type of the ferrocyanides and ferricyanides and their platinum analogues. Such a salt as K2PtCy4may accordingly be regarded as Pt(Cy2K)2, like Pt(OH)2; and such a salt as PtK2Cy5as PtCy(Cy2K)2, the analogue of PtX(OH)2, or AlX(OH)2, and other compounds of the type RX3. Potassium ferricyanide and the analogous compounds of cobalt, iridium, and rhodium, belong to the same type, with the same difference as there is between RX(OH)2and R(OH)3, since FeK3Cy6= Fe(Cy2K)3. Limiting myself to these considerations, which may partially elucidate the nature of double salts, I will now pass again to the complex saline compounds known for platinum.(A) On mixing a solution of potassium thiocyanate with a solution of potassium platinosochloride, K2PtCl4, they form a double thiocyanate, PtK2(CNS)4, which is easily soluble in water and alcohol, crystallises in red prisms, and gives an orange-coloured solution, which precipitates salts of the heavy metals. The action of sulphuric acid on the lead salt of the same type gives the acid itself, PtH2(SCN)4, which corresponds with these salts. The type of these compounds is evidently the same as that of the cyanides.(B)Platinous chloride, PtCl2, which is insoluble in water, formsdouble salts with the metallic chlorides. These double chlorides are soluble in water, and capable of crystallising. Hence when a hydrochloric acid solution of platinous chloride is mixed with solutions of metallic salts and evaporated it forms crystalline salts of a red or yellow colour. Thus, for example, the potassium salt, PtK2Cl4, is red, and easily soluble in water; the sodium salt is also soluble in alcohol; the barium salt, PtBaCl4,3H2O, is soluble in water, but the silver salt, PtAg2Cl4, is insoluble in water, and may be used for obtaining the remaining salts by means of double decomposition with their chlorides.(C) A remarkable example of the complex compounds of platinum was observed by Schützenberger (1868). He showed that finely-divided platinum in the presence of chlorine and carbonic oxide at 250°-300° gives phosgene and a volatile compound containing platinum. The same substance is formed by the action of carbonic oxide on platinous chloride. It decomposes with an explosion in contact with water. Carbon tetrachloride dissolves a portion of this substance, and on evaporation gives crystals of 2PtCl2,3CO, whilst the compound PtCl2,2CO remains undissolved. When fused and sublimed it gives yellow needles of PtCl2,CO, and in the presence of an excess of carbonic oxide PtCl2,2CO is formed. These compounds are fusible (the first at 250°, the second at 142°, and the third at 195°). In this case (as in the double cyanides) combination takes place, because both carbonic oxide and platinous chloride are unsaturated compounds capable of further combination. The carbon tetrachloride solution absorbs NH3and gives PtCl2,CO,2NH3, and PtCl2,2CO,2NH3, and these substances are analogous (Foerster, Zeisel, Jörgensen) to similar compounds containing complex amines (for instance, pyridine, C5H5N), instead of NH3, and ethylene, &c., instead of CO, so that here we have a whole series of complex platino-compounds. The compound PtCl2CO dissolves in hydrochloric acid without change, and the solution disengages all the carbonic oxide when KCN is added to it, which shows that those forces which bind 2 molecules of KCN to PtCl2can also bind the molecule CO, or 2 molecules of CO. When the hydrochloric acid solution of PtCl2CO is mixed with a solution of sodium acetate or acetic acid, it gives a precipitate of PtOCO,i.e.the Cl2is replaced by oxygen (probably because the acetate is decomposed by water). This oxide, PtOCO, splits up into Pt + CO2at 350°. PtSCO is obtained by the action of sulphuretted hydrogen upon PtCl2CO. All this leads to the conclusion that the group PtCO is able to assimilate X2= Cl2, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by igniting spongy platinum at 250°, first in a stream of chlorine, and then in a stream of carbonic oxide, obtained (besides volatile products) a non-volatile yellow substance which remained unchanged in air and disengaged chlorine and phosgene gas when ignited; its composition was PtCl6(CO)2, which apparently proves it to be a compound of PtCl2and 2COCl2, as PtCl2is able to combine with oxychlorides, and forms somewhat stable compounds.(D) The faculty of platinous chloride for forming stable compounds with divers substances shows itself in the formation of the compound PtCl2,PCl3by the action of phosphorus pentachloride at 250° on platinum powder (Pd reacts in a similar manner, according to Fink, 1892). The product contains both phosphorus pentachloride and platinum, whilst the presence of PtCl2is shown in the fact that the action of water produceschlorplatino-phosphorous acid, PtCl2P(OH)3.(E) After the cyanides, thedouble saltsof platinumformed by sulphurous acidare most distinguished for their stability and characteristic properties. This is all the more instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as in all its compounds, it exhibits a dual reaction. The salts of sulphurous acid, R2SO3, either react as salts of a feeble bibasic acid, where the group SO3presents itself as bivalent, and consequently equal to X2, or else they react after the manner of salts of a monobasic acid containing the same residue, RSO3, as occurs in the salts of sulphuric acid. In sulphurous acid this residue is combined with hydrogen, H(SO3H), whilst in sulphuric acid it is united with the aqueous residue (hydroxyl), OH(SO3H). These two forms of action of the sulphites appear in their reactions with the platinum salts—that is to say, salts of both kinds are formed, and they both correspond with the type PtH2X4. The one series of salts contain PtH2(SO3)2, and their reactions are due to the bivalent residue of sulphurous acid, which replaces X2. The others, which have the composition PtR2(SO3H)4, contain sulphoxyl. The latter salts will evidently react like acids; they are formed simultaneously with the salts of the first kind, and pass into them. These salts are obtained either by directly dissolving platinous oxide in water containing sulphurous acid, or by passing sulphurous anhydride into a solution of platinous chloride in hydrochloric acid. If a solution of platinous chloride or platinous oxide in sulphurous acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate containing PtNa2(SO3Na)4,7H2O. If this precipitate be dissolved in a small quantity of hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of the other type, PtNa2(SO3)2,H2O, in the form of a yellow powder, which is sparingly soluble in water. The potassium salt analogous to the first salt, PtK2(SO3K)4,2H2O, is precipitated by passing sulphurous anhydride into a solution of potassium sulphite in which platinous oxide is suspended. A similar salt is known for ammonium, and with hydrochloric acid it gives a salt of the second kind, Pt(NH4)2(SO3)2,H2O. If ammonio-chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first deoxidised, and chlorine is evolved, forming a salt of the type PtX2; a double decomposition then takes place with the ammonium sulphite, and a salt of the composition Pt(NH4)2Cl3(SO3H) is formed (in a desiccator). The acid character of this substance is explained by the fact that it contains the elements SO3H—sulphoxyl, with the hydrogen not yet displaced by a metal. On saturating a solution of this acid with potassium carbonate it gives orange-coloured crystals of a potassium salt of the composition Pt(NH4)2Cl3(SO3K). Here it is evident that an equivalent of chlorine in Pt(NH4)2Cl4is replaced by the univalent residue of sulphurous acid. Among these salts, that of the composition Pt(NH4)Cl2(SO3H)2,H2O is very readily formed, and crystallises in well-formed colourless crystals; it is obtained by dissolving ammonium platinosochloride, Pt(NH4)2Cl4, in an aqueous solution of sulphurous acid. The difficulty with which sulphurous anhydride and platinum are separated from these salts indicates the same basic character in these compounds as is seen in the double cyanides of platinum. In their passage into a complex salt, the metal platinum and the group SO2modify their relations (compared with those of PtX2or SO2X2), just as the chlorine in the salts KClO, KClO3, and KClO4is modified in its relations as compared with hydrochloric acid or potassium chloride.(F) No less characteristic are theplatinonitritesformed by platinous oxide. They correspond with nitrous acid, whose salts, RNO2, contain the univalent radicle, NO2, which is capable of replacing chlorine, and therefore the salts of this kind should form a common type PtR2(NO2)4, and such a salt of potassium has actually been obtained by mixing a solution of potassium platinosochloride with a solution of potassium nitrite, when the liquid becomes colourless, especially if it be heated, which indicates the change in the chemical distribution of the elements. As the liquid decolorises it gradually deposits sparingly soluble, colourless prisms of the potassium salt K2Pt(NO2)4, which does not contain any water. With silver nitrate a solution of this salt gives a precipitate of silver platinonitrite, PtAg2(NO2)4. The silver of this salt may be replaced by other metals by means of double decomposition with metallic chlorides. The sparingly soluble barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble acid, which separates, under the receiver of an air-pump, in red crystals; this acid has the composition PtH2(NO2)4. To the potassium salt, K2Pt(NO2)4, there correspond (Vèzes, 1892) K2Pt(NO2)4Br2and K2Pt(NO2)4Cl2and other compounds of the same type K2PtX6, where X is partly replaced by Cl or Br and partly by (NO2), showing a transition towards the type of the double salts like the platino-ammoniacal salts. (The corresponding double sodium nitrite salt of cobalt is soluble in water, while the K,NH4and many other salts are insoluble in water, as I was informed by Prof. K. Winkler in 1894).In all the preceding complex compounds of Pt we see a common type PtX2,2MX (i.e.of double salts corresponding to PtO) or PtM2X4= Pt(MX2)2, corresponding to Pt(HO)2with the replacement of O by its equivalent X2. Two other facts must also be noted. In the first place these X's generally correspond to elements (like chlorine) or groups (like CN, NO2, SO3, &c.), which are capable of further combination. In the second place all the compounds of the type PtM2X4are capable of combining with chlorine or similar elements, and thus passing into compounds of the types PtX3or PtX4.[12]The platinum salt and ammonia, when once combined together, are no longer subject to their ordinary reactions but form compounds which are comparatively very stable. The question at once suggests itself to all who are acquainted with these phenomena, as to what is the relation of the elements contained in these compounds. The first explanation is that these compounds are salts of ammonium in which the hydrogen is partially replaced by platinum. This is the view, with certain shades of difference, held by many respecting the platino-ammonium compounds. They were regarded in this light by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we suppose the hydrogen in 2NH4X to be replaced by bivalent platinum (as in the salts PtX2), we shall obtainNH3NH3PtXX—that is, the compound PtX2,2NH3. The compound with 4NH3will then be represented by a further substitution of the hydrogen in ammonia by ammonium itself—i.e.as NH2(NH4X)2Pt or PtX2,4NH3. A modification of this view is found in that representation of compounds of this kind which is based on atomicity. As platinum in PtX2is bivalent, has two affinities, and ammonia, NH3, is also bivalent, because nitrogen is quinquivalent and is here only combined with H3, it is evident what bonds should be represented in PtX2,2NH3and in PtX2,4NH3. In the former, Pt(NH3Cl)2, the nitrogen of each atom of ammonia is united by three affinities with H3, by one with platinum, and by the fifth with chlorine. The other compound is Pt(NH3.NH3Cl)2—that is, the N is united by one affinity with the other N, whilst the remaining bonds are the same as in the first salt. It is evident that this union or chain of ammonias has no obvious limit, and the most essential fault of such a mode of representation is that it does not indicate at all what number of ammonias are capable of being retained by platinum. Moreover, it is hardly possible to admit the bond between nitrogen and platinum in such stable compounds, for these kinds of affinities are, at all events, feeble, and cannot lead to stability, but would rather indicate explosive and easily-decomposed compounds. Moreover, it is not clear why this platinum, which is capable of giving PtX4, does not act with its remaining affinities when the addition of ammonia to PtX2takes place. These, and certain other considerations which indicate the imperfection of this representation of the structure of the platino-ammonium salts, cause many chemists to incline more to the representations of Berzelius, Claus, Gibbs, and others, who suppose that NH3is able to combine with substances, to adjoin itself or pair itself with them (this kind of combination is called ‘Paarung’) without altering the fundamental capacity of a substance for further combinations. Thus, in PtX2,2NH3, the ammonia is the associate of PtX2, as is expressed by the formula N2H6PtX2. Without enlarging on the exposition of the details of this doctrine, we will only mention that it, like the first, does not render it possible to foresee a limit to the compounds with ammonia; it isolates compounds of this kind into a special and artificial class; does not show the connection between compounds of this and of other kinds, and therefore it essentially only expresses the fact of the combination with ammonia and the modification in its ordinary reactions. For these reasons we do not hold to either of these proposed representations of the ammonio-platinum compounds, but regard them from the point of view cited above with reference to double salts and water of crystallisation—that is, we embrace all these compounds under the representation of compounds of complex types. The type of the compound PtX2,2NH3is far more probably the same as that of PtX2,2Z—i.e.as PtX4, or, still more accurately and truly, it is a compound of the same type as PtX2,2KX or PtX2,2H2O, &c. Although the platinum first entered into PtK2X4as the type PtX2, yet its character has changed in the same manner as the character of sulphur changes when from SO2the compound SO2(OH)2is obtained, or when KClO4, the higher form, is obtained from KCl. For us as yet there is no question as towhataffinities hold X2and what hold 2NH3, because this is a question which arises from the supposition of the existence of different affinities in the atoms, which there is no reason for taking as a common phenomenon. It seems to me that it is most importantas a commencementto render clear the analogy in the formation of various complex compounds, and it is this analogy of the ammonia compounds with those of water of crystallisation and double salts that forms the main object of the primary generalisation. We recognise in platinum, at all events, not only the four affinities expressed in the compound PtCl4, but a much larger number of them, if only thesummation of affinitiesis actually possible. Thus, in sulphur we recognise not two but a much greater number of affinities; it is clear that at least six affinities can act. So also among the analogues of platinum: osmic anhydride, OsO4, Ni(CO)4, PtH2Cl6, &c. indicate the existence of at least eight affinities; whilst, in chlorine, judging from the compound KClO4= ClO3(OK) = ClX7, we must recognise at least seven affinities, instead of the one which is accepted. The latter mode of calculating affinities is a tribute to that period of the development of science when only the simplest hydrogen compounds were considered, and when all complex compounds were entirely neglected (they were placed under the class of molecular compounds). This is insufficient for the present state of knowledge, because we find that, in complex compounds as in the most simple, the same constant types or modes of equilibrium are repeated, and the character of certain elements is greatly modified in the passage from the most simple into very complex compounds.Judging from the most complex platino-ammonium compounds PtCl4,4NH3, we should admit the possibility of the formation of compounds of the type PtX4Y4, where Y4= 4X2= 4NH3, and this shows that those forces which form such a characteristic series of double platinocyanides PtK2(CN)4,3H2O, probably also determine the formation of the higher ammonia derivatives, as is seen on comparing—PtCl2NH32Cl23NH3Pt(CN)2KCNKCN3H2O.Moreover, it is obviously much more natural to ascribe the faculty for combination withnY to the whole of the acting elements—that is, to PtX2or PtX4, and not to platinum alone. Naturally such compounds are not produced with any Y. With certain X's there only combine certain Y's. The best known and most frequently-formed compounds of this kind are those with water—that is, compounds with water of crystallisation. Compounds with salts are double salts; also we know that similar compounds are also frequently formed by means of ammonia. Salts of zinc, ZnX2, copper, CuX2, silver, AgX, and many others give similar compounds, but these and many otherammonio-metallicsaline compounds are unstable, and readily part with their combined ammonia, and it is only in the elements of the platinum group and in the group of the analogues of iron, that we observe the faculty to form stable ammonio-metallic compounds. It must be remembered that the metals of the platinum and iron groups are able to form several high grades of oxidation which have an acid character, and consequently in the lower degrees of combination there yet remain affinities capable of retaining other elements, and they probably retain ammonia, and hold it the more stably, because all the properties of the platinum compounds are rather acid than basic—that is, PtXnrecalls rather HX or SnXnor CXnthan KX, CaX2, BaX2, &c., and ammonia naturally will rather combine with an acid than with a basic substance. Further, a dependence, or certain connection of the forms of oxidation with the ammonia compounds, is seen on comparing the following compounds:PdCl2,2NH3,H2OPdCl2,4NH3,H2OPtCl2,2NH3PtCl4,4NH3RhCl3,5NH3RuCl2,4NH3,3H2OIrCl3,5NH3OsCl2,4NH3,2H2OWe know that platinum and palladium give compounds of lower types than iridium and rhodium, whilst ruthenium and osmium give the highest forms of oxidation; this shows itself in this case also. We have purposely cited the same compounds with 4NH3for osmium and ruthenium as we have for platinum and palladium, and it is then seen that Ru and Os are capable of retaining 2H2O and 3H2O, besides Cl2and NH3, which the compounds of platinum and palladium are unable to do. The same ideas which were developed in Note35, Chapter XXII. respecting the cobaltia compounds are perfectly applicable to the present case,i.e.to theplatiniacompounds or ammonia compounds of the platinum metals, among which Rh and Ir give compounds which are perfectly analogous to the cobaltia compounds.Iridium and rhodium, which easily give compounds of the type RX3, give compounds (Claus) of the type IrX3,5NH3, of a rose colour, and RhX3,5NH3, of a yellow colour. Jörgensen, in his researches on these compounds, showed their entire analogy with the cobalt compounds, as was to be expected from the periodic system.[13]Subsequently, a whole series of such compounds was obtained with various elements in the place of the (non-reacting) chlorine, and nevertheless they, like the chlorine, reacted with difficulty, whilst the second portion of the X's introduced into such salts easily underwent reaction. This formed the most important reason for the interest which the study of the composition and structure of the platino-ammonium salts subsequently presented to many chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt, Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and others. The salts PtX4,2NH3, discovered by Gerhardt, also exhibited several different properties in the two pairs of X's. In the remaining platino-ammonium salts all the X's appear to react alike.The quality of the X's, retainable in the platino-ammonium salts, may be considerably modified, and they may frequently be wholly or partially replaced by hydroxyl. For example, the action of ammonia on the nitrate of Gerhardt's base, Pt(NO3)4,2NH3, in a boiling solution, gradually produces a yellow crystalline precipitate which is nothing else than abasic hydrateoralkali, Pt(OH)4,2NH3. It is sparingly soluble in water, but gives directly soluble salts PtX4,2NH3with acids. The stability of this hydroxide is such that potash does not expel ammonia from it, even on boiling, and it does not change below 130°. Similar properties are shown by the hydroxide Pt(OH)2,2NH3and the oxide PtO,2NH3of Reiset's second base. But the hydroxides of the compounds containing 4NH3are particularly remarkable. The presence of ammonia renders them soluble and energetic. The brevity of this work does not permit us, however, to mention many interesting particulars in connection with this subject.[14]Hydroxides are known corresponding with Gros's salts, which contain one hydroxyl group in the place of that chlorine or haloid which in Gros's salts reacts with difficulty, and these hydroxides do not at once show the properties of alkalis, just as the chlorine which stands in the same place does not react distinctly; but still, after the prolonged action of acids, this hydroxyl group is also replaced by acids. Thus, for example, the action of nitric acid on Pt(NO3)2Cl2,4NH3causes the non-active chlorine to react, but in the product all the chlorine is not replaced by NO3, but only half, and the other half is replaced by the hydroxyl group: Pt(NO3)2Cl2,4NH3+ HNO3+ H2O = Pt(NO3)3(OH),4NH3+ 2HCl; and this is particularly characteristic, because here the hydroxyl group has not reacted with the acid—an evident sign of the non-alkaline character of this residue. I think it may be well to call attention to the fact that the composition of the ammonio-metallosalts very often exhibits a correspondence between the amount of X's and the amount of NH3, of such a nature that we find they contain either XNH3or the grouping X2NH3; for example, Pt(XNH3)2and Pt(X2NH3)2, Co(X2NH3)3, Pt(XNH3)4, &c. Judging from this, the view of the constitution of the double cyanides of platinum given in Note11finds some confirmation here, but, in my opinion, all questions respecting the composition (and structure) of the ammoniacal, double, complex, and crystallisation compounds stand connected with the solution of questions respecting the formation of compounds of various degrees of stability, among which a theory of solutions must be included, and therefore I think that the time has not yet come for a complete generalisation of the data which exist for these compounds; and here I again refer the reader to Prof. Kournakoff's work cited in Chapter XXII., Note35. However, we may add a few individual remarks concerning the platinia compounds.To the common properties of the platino-ammonium salts, we must add not only theirstability(feeble acids and alkalis do not decompose them, the ammonia is not evolved by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed in them to as great an extent as those of iron in the ferricyanides. Thus neither alkalis nor hydrogen sulphide will separate the platinum from them. For example, sulphuretted hydrogen in acting on Gros's salts gives sulphur, removes half the chlorine by means of its hydrogen, and forms salts of Reiset's first base. This may be understood or explained by considering the platinum in the molecule as covered, walled up by the ammonia, or situated in the centre of the molecule, and therefore inaccessible to reagents. On this assumption, however, we should expect to find clearly-expressed ammoniacal properties, and this is not the case. Thus ammonia is easily decomposed by chlorine, whilst in acting on the platino-ammonium salts containing PtX2and 2NH3or 4NH3, chlorine combines and does not destroy the ammonia; it converts Reiset's salts into those of Gros and Gerhardt. Thus from PtX2,2NH3there is formed PtX2Cl2,2NH3, and from PtX2,4NH3the salt of Gros's base PtX2Cl2,4NH3. This shows that the amount of chlorine which combines is not dependent on the amount of ammonia present, but is due to the basic properties of platinum. Owing to this some chemists suppose the ammonia to be inactive or passive in certain compounds. It appears to me that these relations, these modifications, in the usual properties of ammonia and platinum are explained directly by their mutual combination. Sulphur, in sulphurous anhydride, SO2, and hydrogen sulphide, SH2, is naturally one and the same, but if we only knew of it in the form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride, we should consider its properties as hidden. The oxygen in magnesia, MgO, and in nitric peroxide, NO2, is so different that there is no resemblance. Arsenic no longer reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and in their compounds with nitrogen all metals modify both their reactions and their physical properties. We are accustomed to judge the metals by their saline compounds with haloid groups, and ammonia by its compounds with acid substances, and here, in the platino-compounds, if we assume the platinum to be bound to the entire mass of the ammonia—to its hydrogen and nitrogen—we shall understand that both the platinum and ammonia modify their characters. Far more complicated is the question why a portion of the chlorine (and other haloid simple and complex groups) in Gros's salts acts in a different manner from the other portion, and why only half of it acts in the usual way. But this also is not an exclusive case. The chlorine in potassium chlorate or in carbon tetrachloride does not react with the same ease with metals as the chlorine in the salts corresponding with hydrochloric acid. In this case it is united to oxygen and carbon, whilst in the platino-ammonium compounds it is united partly to platinum and partly to the platino-ammonium group. Many chemists, moreover, suppose that a part of the chlorine is united directly to the platinum and the other part to the nitrogen of the ammonia, and thus explain the difference of the reactions; but chlorine united to platinum reacts as well with a silver salt as the chlorine of ammonium chloride, NH4Cl, or nitrosyl chloride, NOCl, although there is no doubt that in this case there is a union between the chlorine and nitrogen. Hence it is necessary to explain the absence of a facile reactive capacity in a portion of the chlorine by the conjoint influence of the platinum and ammonia on it, whilst the other portion may be admitted as being under the influence of the platinum only, and therefore as reacting as in other salts. By admitting a certain kind of stable union in the platino-ammonium grouping, it is possible to imagine that the chlorine does not react with its customary facility, because access to a portion of the atoms of chlorine in this complex grouping is difficult, and the chlorine union is not the same as we usually meet in the saline compounds of chlorine. These are the grounds on which we, in refuting the now accepted explanations of the reactions and formation of the platino-compounds, pronounce the following opinion as to their structure.In characterising the platino-ammonium compounds, it is necessary to bear in mind that compounds which already contain PtX4do not combine directly with NH3, and that such compounds as PtX4,4NH3only proceed from PtX2, and therefore it is natural to conclude that those affinities and forces which cause PtX2to combine with X2also cause it to combine with 2NH3. And having the compound PtX2,2NH3, and supposing that in subsequently combining with Cl2it reacts with those affinities which produce the compounds of platinic chloride, PtCl4, with water, potassium chloride, potassium cyanide, hydrochloric acid, and the like, we explain not only the fact of combination, but also many of the reactions occurring in the transition of one kind of platino-ammonium salts into another. Thus by this means we explain the fact that (1) PtX2,2NH3combines with 2NH3, forming salts of Reiset's first base; (2) and the fact that this compound (represented as follows for distinctness), PtX2,2NH3,2NH3, when heated, or even when boiled in solution, again passes into PtX2,2NH3(which resembles the easy disengagement of water of crystallisation, &c.); (3) the fact that PtX2,2NH3is capable of absorbing, under the action of the same forces, a molecule of chlorine, PtX2,2NH3,Cl2, which it then retains with energy, because it is attracted, not only by the platinum, but also by the hydrogen of the ammonia; (4) the fact that this chlorine held in this compound (of Gerhardt) will have a position unusual in salts, which will explain a certain (although very feebly-marked) difficulty of reaction; (5) the fact that this does not exhaust the faculty of platinum for further combination (we need only recall the compound PtCl4,2HCl,16H2O), and that therefore both PtX2,2NH3,Cl2and PtX2,2NH3,2NH3are still capable of combination, whence the latter, with chlorine, gives PtX2,2NH3,2NH3,Cl2, after the type of PtX4Y4(and perhaps higher); (6) the fact that Gros's compounds thus formed are readily reconverted into the salts of Reiset's first base when acted on by reducing agents; (7) the fact that in Gros's salts, PtX2,2NH3(NH3X)2, the newly-attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is attached both to the platinum and to the ammonia, for both of which it has an attraction; (8) the fact that the faculty for further combination is not even yet exhausted in the type of Gros's salts, and that we actually have a compound of Gros's chlorine salt with platinous chloride and with platinic chloride; the salt PtSO4,2NH3,2NH3,SO4combines further also with H2O; (9) the fact that such a faculty of combination with new molecules is naturally more developed in the lower forms of combination than in the higher. Hence the salts of Reiset's first base—for example, PtCl2,2NH3,2NH3—both combine with water and give precipitates (soluble in water but not in hydrochloric acid) of double salts with many salts of the heavy metals—for example, with lead chloride, cupric chloride, and also with platinic and platinous chlorides (Buckton's salts). The latter compounds will have the composition PtCl2,2NH3,2NH3,PtCl2—that is, the same composition as the salts of Reiset's second base, but it cannot be identical with it. Such an interesting case does actually exist. The first salt, PtCl2,4NH3,PtCl2, is green, insoluble in water and in hydrochloric acid, and is known asMagnus's salt, and the second, PtCl2,2NH3, is Reiset's yellow, sparingly soluble (in water). They are polymeric, namely, the first contains twice the number of elements held in the second, and at the same time they easily pass into each other. If ammonia be added to a hot hydrochloric acid solution of platinous chloride, it forms the salt PtCl2,4NH3, but in the presence of an excess of platinous chloride it gives Magnus's salt. On boiling the latter in ammonia it gives a colourless soluble salt of Reiset's first base, PtCl2,4NH3, and if this be boiled with water, ammonia is disengaged, and a salt of Reiset's second base, PtCl2,2NH3, is obtained.A class of platino-ammonium isomerides (obtained by Millon and Thomsen) are also known. Buckton's salts—for example, the copper salt—were obtained by them from the salts of Reiset's first base, PtCl2,4NH3, by treatment with a solution of cupric chloride, &c., and therefore, according to our method of expression, Buckton's copper salt will be PtCl2,4NH3,CuCl2. This salt is soluble in water, but not in hydrochloric acid. In it the ammonia must be considered as united to the platinum. But if cupric chloride be dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is added to it, a violet precipitate is obtained of the same composition as Buckton's salt, which, however, is insoluble in water, but soluble in hydrochloric acid. In this a portion, if not all, of the ammonia must be regarded as united to the copper, and it must therefore be represented as CuCl2,4NH3,PtCl2. This form is identical in composition but different in properties (is isomeric) with the preceding salt (Buckton's). The salt of Magnus is intermediate between them, PtCl2,4NH3,PtCl2; it is insoluble in water and hydrochloric acid. These and certain other instances of isomeric compounds in the series of the platino-ammonium salts throw a light on the nature of the compounds in question, just as the study of the isomerides of the carbon compounds has served and still serves as the chief cause of the rapid progress of organic chemistry. In conclusion, we may add that (according to the law of substitution) we must necessarily expect all kinds of intermediate compounds between the platino and analogous ammonia derivatives on the one hand, and the complex compounds of nitrous acid on the other. Perhaps the instance of the reaction of ammonia upon osmic anhydride, OsO4, observed by Fritsche, Frémy, and others, and more fully studied by Joly (1891), belongs to this class. The latter showed that when ammonia acts upon an alkaline solution of OsO4the reaction proceeds according to the equation: OsO4+ KHO + NH3= OsNKO3+ 2H2O. It might be imagined that in this case the ammonia is oxidised, probably forming the residue of nitrous acid (NO), while the type OsO4is deoxidised into OsO2, and a salt, OsO(NO)(KO), of the type OsX4is formed. This salt crystallises well in light yellow octahedra. It corresponds toosmiamic acid, OsO(ON)(HO), whose anhydride [OsO(NO)]2, has the composition Os2N2O5, which equals 2Os + N2O5to the same extent as the above-mentioned compound PtCO2equals Pt + CO2(seeNote11).
[11]This acid character is explained by the influence of the platinum on the hydrogen, and by the attachment of the cyanogen groups. Thus cyanuric acid, H3(CN)3O3, is an energetic acid compared with cyanic acid, HCNO. And the formation of a compound with five molecules of water of crystallisation, (PtH2(CN)4,5H2O), confirms the opinion that platinum is able to form compounds of still higher types than that expressed in its saline compounds, and, moreover, the combination of hydroplatinocyanic acid with water does not reach the limit of the compounds which appears in PtCl4,2HCl,6H2O.A whole series ofplatinocyanidesof the common type PtR2(CN)4nH2O is obtained by means of double decomposition with the potassium or hydrogen or silver salts. For example, the salts of sodium and lithium contain, like the potassium salt, three molecules of water. The sodium salt is soluble in water and alcohol. The ammonium salt has the composition Pt(NH4)2(CN)4,2H2O and gives crystals which reflect blue and rose-coloured light. This ammonium salt decomposes at 300°, with evolution of water and ammonium cyanide, leaving a greenishplatinum dicyanide, Pt(CN)2, which is insoluble in water and acid but dissolves in potassium cyanide, hydrocyanic acid, and other cyanides. The same platinous cyanide is obtained by the action of sulphuric acid on the potassium salts in the form of a reddish-brown amorphous precipitate. The most characteristic of the platinocyanides are those of the alkaline earths. The magnesium salt PtMg(CN)4,7H2O crystallises in regular prisms, whose side faces are of a metallic green colour and terminal planes dark blue. It shows a carmine-red colour along the main axis, and dark red along the lateral axes; it easily loses water, (2H2O), at 40°, and then turns blue (it then contains 5H2O, which is frequently the case with the platinocyanides). Its aqueous solution is colourless, and an alcoholic solution deposits yellow crystals. The remainder of the water is given off at 230°. It is obtained by saturating platinocyanic acid with magnesia, or else by double decomposition between the barium salt and magnesium sulphate. The strontium salt, SrPt(CN)4,4H2O crystallises in milk-white plates having a violet and green iridescence. When it effloresces in a desiccator, its surfaces have a violet and metallic green iridescence. A colourless solution of the barium salt PtBa(CN)4,4H2O is obtained by saturating a solution of hydroplatinocyanic acid with baryta, or by boiling the insoluble copper platinocyanide in baryta water. It crystallises in monoclinic prisms of a yellow colour, with blue and green iridescence; it loses half its water at 100°, and the whole at 150°. The ethyl salt, Pt(C2H5)2(CN)4,2H20, is also very characteristic; its crystals are isomorphous with those of the potassium salt, and are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatinocyanic acid. The facility with which they crystallise, the regularity of their forms, and their remarkable play of colours, renders the preparation of the platinocyanides one of the most attractive lessons of the laboratory.By the action of chlorine or dilute nitric acid, the platinocyanides are converted into salts of the composition PtM2(CN)5, which corresponds with Pt(CN)3,2KCN—that is, they express the type of a non-existent form of oxidation of platinum, PtX3(i.e.oxide Pt2O3), just as potassium ferricyanide (FeCy3,3KCy) corresponds with ferric oxide, and the ferrocyanide corresponds with the ferrous oxide. The potassium salt of this series contains PtK2(CN)5,3H2O, and forms brown regular prisms with a metallic lustre, and is soluble in water but insoluble in alcohol. Alkalis re-convert this compound into the ordinary platinocyanide K2Pt(CN)4, taking up the excess of cyanogen. It is remarkable that the salts of the type PtM2Cy5contain the same amount of water of crystallisation as those of the type PtM2Cy4. Thus the salts of potassium and lithium contain three, and the salt of magnesium seven, molecules of water, like the corresponding salts of the type of platinous oxide. Moreover, neither platinum nor any of its associates gives any cyanogen compound corresponding with the oxide,i.e.having the composition PtK2Cy6, just as there are no compounds higher than those which correspond to RCy3nMCy3for cobalt or iron. This would appear to indicate the absence of any such cyanides, and indeed, for no element are there yet known any poly-cyanides containing more than three equivalents of cyanogen for one equivalent of the element. The phenomenon is perhaps connected with the faculty of cyanogen of giving tricyanogen polymerides, such as cyanuric acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution of PtK2(CN)4gives (besides PtK2Cy5) a product PtK2Cy4Cl2, which evidently contains the form PtX4, but at first the action of the chlorine (or the electrolysis of, or addition of dilute peroxide of hydrogen to, a solution of PtK2Cy4, acidulated with hydrochloric acid) produces an easily soluble intermediate salt which crystallises in thin copper-red needles (Wilm, Hadow, 1889). It only contains a small amount of chlorine, and apparently corresponds to a compound 5PtK2Cy4+ PtK2Cy4Cl2+ 24H2O. Under the action of an excess of ammonia both these chlorine products are converted either completely or in part (according to Wilm ammonia does not act upon PtK2Cy4) into PtCy2,2NH3,i.e.a platino-ammonia compound (seefurther on). It is also necessary to pay attention to the fact that ruthenium and osmium—which, as we know, give higher forms of oxidation than platinum—are also able to combine with a larger proportion of potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a crystallinehydroruthenocyanic acid, RuH4(CN)6, which is soluble in water and alcohol, and corresponds with the salts M4Ru(CN)6. There are exactly similar osmic compounds—for example, K4Os(CN)6,3H2O. The latter is obtained in the form of colourless, sparingly-soluble regular tablets on evaporating the solution obtained from a fused mixture of potassium osmiochloride, K2OsCl6, and potassium cyanide. These osmic and ruthenic compounds fully correspond with potassium ferrocyanide, K4Fe(CN)6,3H2O, not only in their composition but also in their crystalline form and reactions, which again demonstrates the close analogy between iron, ruthenium, and osmium, which we have shown by giving these three elements a similar position (in the eighth group) in the periodic system. For rhodium and iridium only salts of the same type as the ferricyanides, M3RCy6, are known, and for palladium only of the type M2PdCy4, which are analogous to the platinum salts. In all these examples aconstancy of the typesof the double cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, gold. The double cyanides of iron, ruthenium, osmium have the type K4R(CN)6; of cobalt, rhodium, iridium, the type K3R(CN)6; of nickel, palladium, platinum the type K2R(CN)4and K2R(CN)5; and for copper, silver, gold there are known KR(CN)2, so that the presence of 4, 3, 2, and 1 atoms of potassium corresponds with the order of the elements in the periodic system. Those types which we have seen in the ferrocyanides and ferricyanides of iron repeat themselves in all the platinoid metals, and this naturally leads to the conclusion that the formation of similar so-called double salts is of exactly the same nature as that of the ordinary salts. If, in expressing the union of the elements in the oxygen salts, the existence of anaqueous residue(hydroxyl group) be admitted, in which the hydrogen is replaced by a metal, we have then only to apply this mode of expression to the double salts and the analogy will be obvious, if only we remember that Cl2, (CN)2, SO4, &c., are equivalent to O, as we see in RO, RCl2, RSO4, &c. They all = X2, and, therefore, in point of fact, wherever X (= Cl or OH, &c.) can be placed, there (Cl2H), (SO4H), &c., can also stand. And as Cl2H = Cl + HCl and SO4H = OH + SO3, &c., it follows that molecules HCl or SO3, or, in general, whole molecules—for instance, NH3, H2O, salts, &c., can annex themselves to a compound containing X. (This is an indirect consequence of the law of substitution which explains the origin of double salts, ammonia compounds, compounds with water of crystallisation, &c., by one general method.) Thus the double salt MgSO4,K2SO4, according to this reasoning,may beconsidered as a substance of the same type as MgCl2, namely, as = Mg(SO4K)2, and the alums as derived from Al(OH)(SO4), namely, as Al(SO4K)(SO4). Without stopping to pursue this digression further, we will apply these considerations to the type of the ferrocyanides and ferricyanides and their platinum analogues. Such a salt as K2PtCy4may accordingly be regarded as Pt(Cy2K)2, like Pt(OH)2; and such a salt as PtK2Cy5as PtCy(Cy2K)2, the analogue of PtX(OH)2, or AlX(OH)2, and other compounds of the type RX3. Potassium ferricyanide and the analogous compounds of cobalt, iridium, and rhodium, belong to the same type, with the same difference as there is between RX(OH)2and R(OH)3, since FeK3Cy6= Fe(Cy2K)3. Limiting myself to these considerations, which may partially elucidate the nature of double salts, I will now pass again to the complex saline compounds known for platinum.(A) On mixing a solution of potassium thiocyanate with a solution of potassium platinosochloride, K2PtCl4, they form a double thiocyanate, PtK2(CNS)4, which is easily soluble in water and alcohol, crystallises in red prisms, and gives an orange-coloured solution, which precipitates salts of the heavy metals. The action of sulphuric acid on the lead salt of the same type gives the acid itself, PtH2(SCN)4, which corresponds with these salts. The type of these compounds is evidently the same as that of the cyanides.(B)Platinous chloride, PtCl2, which is insoluble in water, formsdouble salts with the metallic chlorides. These double chlorides are soluble in water, and capable of crystallising. Hence when a hydrochloric acid solution of platinous chloride is mixed with solutions of metallic salts and evaporated it forms crystalline salts of a red or yellow colour. Thus, for example, the potassium salt, PtK2Cl4, is red, and easily soluble in water; the sodium salt is also soluble in alcohol; the barium salt, PtBaCl4,3H2O, is soluble in water, but the silver salt, PtAg2Cl4, is insoluble in water, and may be used for obtaining the remaining salts by means of double decomposition with their chlorides.(C) A remarkable example of the complex compounds of platinum was observed by Schützenberger (1868). He showed that finely-divided platinum in the presence of chlorine and carbonic oxide at 250°-300° gives phosgene and a volatile compound containing platinum. The same substance is formed by the action of carbonic oxide on platinous chloride. It decomposes with an explosion in contact with water. Carbon tetrachloride dissolves a portion of this substance, and on evaporation gives crystals of 2PtCl2,3CO, whilst the compound PtCl2,2CO remains undissolved. When fused and sublimed it gives yellow needles of PtCl2,CO, and in the presence of an excess of carbonic oxide PtCl2,2CO is formed. These compounds are fusible (the first at 250°, the second at 142°, and the third at 195°). In this case (as in the double cyanides) combination takes place, because both carbonic oxide and platinous chloride are unsaturated compounds capable of further combination. The carbon tetrachloride solution absorbs NH3and gives PtCl2,CO,2NH3, and PtCl2,2CO,2NH3, and these substances are analogous (Foerster, Zeisel, Jörgensen) to similar compounds containing complex amines (for instance, pyridine, C5H5N), instead of NH3, and ethylene, &c., instead of CO, so that here we have a whole series of complex platino-compounds. The compound PtCl2CO dissolves in hydrochloric acid without change, and the solution disengages all the carbonic oxide when KCN is added to it, which shows that those forces which bind 2 molecules of KCN to PtCl2can also bind the molecule CO, or 2 molecules of CO. When the hydrochloric acid solution of PtCl2CO is mixed with a solution of sodium acetate or acetic acid, it gives a precipitate of PtOCO,i.e.the Cl2is replaced by oxygen (probably because the acetate is decomposed by water). This oxide, PtOCO, splits up into Pt + CO2at 350°. PtSCO is obtained by the action of sulphuretted hydrogen upon PtCl2CO. All this leads to the conclusion that the group PtCO is able to assimilate X2= Cl2, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by igniting spongy platinum at 250°, first in a stream of chlorine, and then in a stream of carbonic oxide, obtained (besides volatile products) a non-volatile yellow substance which remained unchanged in air and disengaged chlorine and phosgene gas when ignited; its composition was PtCl6(CO)2, which apparently proves it to be a compound of PtCl2and 2COCl2, as PtCl2is able to combine with oxychlorides, and forms somewhat stable compounds.(D) The faculty of platinous chloride for forming stable compounds with divers substances shows itself in the formation of the compound PtCl2,PCl3by the action of phosphorus pentachloride at 250° on platinum powder (Pd reacts in a similar manner, according to Fink, 1892). The product contains both phosphorus pentachloride and platinum, whilst the presence of PtCl2is shown in the fact that the action of water produceschlorplatino-phosphorous acid, PtCl2P(OH)3.(E) After the cyanides, thedouble saltsof platinumformed by sulphurous acidare most distinguished for their stability and characteristic properties. This is all the more instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as in all its compounds, it exhibits a dual reaction. The salts of sulphurous acid, R2SO3, either react as salts of a feeble bibasic acid, where the group SO3presents itself as bivalent, and consequently equal to X2, or else they react after the manner of salts of a monobasic acid containing the same residue, RSO3, as occurs in the salts of sulphuric acid. In sulphurous acid this residue is combined with hydrogen, H(SO3H), whilst in sulphuric acid it is united with the aqueous residue (hydroxyl), OH(SO3H). These two forms of action of the sulphites appear in their reactions with the platinum salts—that is to say, salts of both kinds are formed, and they both correspond with the type PtH2X4. The one series of salts contain PtH2(SO3)2, and their reactions are due to the bivalent residue of sulphurous acid, which replaces X2. The others, which have the composition PtR2(SO3H)4, contain sulphoxyl. The latter salts will evidently react like acids; they are formed simultaneously with the salts of the first kind, and pass into them. These salts are obtained either by directly dissolving platinous oxide in water containing sulphurous acid, or by passing sulphurous anhydride into a solution of platinous chloride in hydrochloric acid. If a solution of platinous chloride or platinous oxide in sulphurous acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate containing PtNa2(SO3Na)4,7H2O. If this precipitate be dissolved in a small quantity of hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of the other type, PtNa2(SO3)2,H2O, in the form of a yellow powder, which is sparingly soluble in water. The potassium salt analogous to the first salt, PtK2(SO3K)4,2H2O, is precipitated by passing sulphurous anhydride into a solution of potassium sulphite in which platinous oxide is suspended. A similar salt is known for ammonium, and with hydrochloric acid it gives a salt of the second kind, Pt(NH4)2(SO3)2,H2O. If ammonio-chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first deoxidised, and chlorine is evolved, forming a salt of the type PtX2; a double decomposition then takes place with the ammonium sulphite, and a salt of the composition Pt(NH4)2Cl3(SO3H) is formed (in a desiccator). The acid character of this substance is explained by the fact that it contains the elements SO3H—sulphoxyl, with the hydrogen not yet displaced by a metal. On saturating a solution of this acid with potassium carbonate it gives orange-coloured crystals of a potassium salt of the composition Pt(NH4)2Cl3(SO3K). Here it is evident that an equivalent of chlorine in Pt(NH4)2Cl4is replaced by the univalent residue of sulphurous acid. Among these salts, that of the composition Pt(NH4)Cl2(SO3H)2,H2O is very readily formed, and crystallises in well-formed colourless crystals; it is obtained by dissolving ammonium platinosochloride, Pt(NH4)2Cl4, in an aqueous solution of sulphurous acid. The difficulty with which sulphurous anhydride and platinum are separated from these salts indicates the same basic character in these compounds as is seen in the double cyanides of platinum. In their passage into a complex salt, the metal platinum and the group SO2modify their relations (compared with those of PtX2or SO2X2), just as the chlorine in the salts KClO, KClO3, and KClO4is modified in its relations as compared with hydrochloric acid or potassium chloride.(F) No less characteristic are theplatinonitritesformed by platinous oxide. They correspond with nitrous acid, whose salts, RNO2, contain the univalent radicle, NO2, which is capable of replacing chlorine, and therefore the salts of this kind should form a common type PtR2(NO2)4, and such a salt of potassium has actually been obtained by mixing a solution of potassium platinosochloride with a solution of potassium nitrite, when the liquid becomes colourless, especially if it be heated, which indicates the change in the chemical distribution of the elements. As the liquid decolorises it gradually deposits sparingly soluble, colourless prisms of the potassium salt K2Pt(NO2)4, which does not contain any water. With silver nitrate a solution of this salt gives a precipitate of silver platinonitrite, PtAg2(NO2)4. The silver of this salt may be replaced by other metals by means of double decomposition with metallic chlorides. The sparingly soluble barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble acid, which separates, under the receiver of an air-pump, in red crystals; this acid has the composition PtH2(NO2)4. To the potassium salt, K2Pt(NO2)4, there correspond (Vèzes, 1892) K2Pt(NO2)4Br2and K2Pt(NO2)4Cl2and other compounds of the same type K2PtX6, where X is partly replaced by Cl or Br and partly by (NO2), showing a transition towards the type of the double salts like the platino-ammoniacal salts. (The corresponding double sodium nitrite salt of cobalt is soluble in water, while the K,NH4and many other salts are insoluble in water, as I was informed by Prof. K. Winkler in 1894).In all the preceding complex compounds of Pt we see a common type PtX2,2MX (i.e.of double salts corresponding to PtO) or PtM2X4= Pt(MX2)2, corresponding to Pt(HO)2with the replacement of O by its equivalent X2. Two other facts must also be noted. In the first place these X's generally correspond to elements (like chlorine) or groups (like CN, NO2, SO3, &c.), which are capable of further combination. In the second place all the compounds of the type PtM2X4are capable of combining with chlorine or similar elements, and thus passing into compounds of the types PtX3or PtX4.
[11]This acid character is explained by the influence of the platinum on the hydrogen, and by the attachment of the cyanogen groups. Thus cyanuric acid, H3(CN)3O3, is an energetic acid compared with cyanic acid, HCNO. And the formation of a compound with five molecules of water of crystallisation, (PtH2(CN)4,5H2O), confirms the opinion that platinum is able to form compounds of still higher types than that expressed in its saline compounds, and, moreover, the combination of hydroplatinocyanic acid with water does not reach the limit of the compounds which appears in PtCl4,2HCl,6H2O.
A whole series ofplatinocyanidesof the common type PtR2(CN)4nH2O is obtained by means of double decomposition with the potassium or hydrogen or silver salts. For example, the salts of sodium and lithium contain, like the potassium salt, three molecules of water. The sodium salt is soluble in water and alcohol. The ammonium salt has the composition Pt(NH4)2(CN)4,2H2O and gives crystals which reflect blue and rose-coloured light. This ammonium salt decomposes at 300°, with evolution of water and ammonium cyanide, leaving a greenishplatinum dicyanide, Pt(CN)2, which is insoluble in water and acid but dissolves in potassium cyanide, hydrocyanic acid, and other cyanides. The same platinous cyanide is obtained by the action of sulphuric acid on the potassium salts in the form of a reddish-brown amorphous precipitate. The most characteristic of the platinocyanides are those of the alkaline earths. The magnesium salt PtMg(CN)4,7H2O crystallises in regular prisms, whose side faces are of a metallic green colour and terminal planes dark blue. It shows a carmine-red colour along the main axis, and dark red along the lateral axes; it easily loses water, (2H2O), at 40°, and then turns blue (it then contains 5H2O, which is frequently the case with the platinocyanides). Its aqueous solution is colourless, and an alcoholic solution deposits yellow crystals. The remainder of the water is given off at 230°. It is obtained by saturating platinocyanic acid with magnesia, or else by double decomposition between the barium salt and magnesium sulphate. The strontium salt, SrPt(CN)4,4H2O crystallises in milk-white plates having a violet and green iridescence. When it effloresces in a desiccator, its surfaces have a violet and metallic green iridescence. A colourless solution of the barium salt PtBa(CN)4,4H2O is obtained by saturating a solution of hydroplatinocyanic acid with baryta, or by boiling the insoluble copper platinocyanide in baryta water. It crystallises in monoclinic prisms of a yellow colour, with blue and green iridescence; it loses half its water at 100°, and the whole at 150°. The ethyl salt, Pt(C2H5)2(CN)4,2H20, is also very characteristic; its crystals are isomorphous with those of the potassium salt, and are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatinocyanic acid. The facility with which they crystallise, the regularity of their forms, and their remarkable play of colours, renders the preparation of the platinocyanides one of the most attractive lessons of the laboratory.
By the action of chlorine or dilute nitric acid, the platinocyanides are converted into salts of the composition PtM2(CN)5, which corresponds with Pt(CN)3,2KCN—that is, they express the type of a non-existent form of oxidation of platinum, PtX3(i.e.oxide Pt2O3), just as potassium ferricyanide (FeCy3,3KCy) corresponds with ferric oxide, and the ferrocyanide corresponds with the ferrous oxide. The potassium salt of this series contains PtK2(CN)5,3H2O, and forms brown regular prisms with a metallic lustre, and is soluble in water but insoluble in alcohol. Alkalis re-convert this compound into the ordinary platinocyanide K2Pt(CN)4, taking up the excess of cyanogen. It is remarkable that the salts of the type PtM2Cy5contain the same amount of water of crystallisation as those of the type PtM2Cy4. Thus the salts of potassium and lithium contain three, and the salt of magnesium seven, molecules of water, like the corresponding salts of the type of platinous oxide. Moreover, neither platinum nor any of its associates gives any cyanogen compound corresponding with the oxide,i.e.having the composition PtK2Cy6, just as there are no compounds higher than those which correspond to RCy3nMCy3for cobalt or iron. This would appear to indicate the absence of any such cyanides, and indeed, for no element are there yet known any poly-cyanides containing more than three equivalents of cyanogen for one equivalent of the element. The phenomenon is perhaps connected with the faculty of cyanogen of giving tricyanogen polymerides, such as cyanuric acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution of PtK2(CN)4gives (besides PtK2Cy5) a product PtK2Cy4Cl2, which evidently contains the form PtX4, but at first the action of the chlorine (or the electrolysis of, or addition of dilute peroxide of hydrogen to, a solution of PtK2Cy4, acidulated with hydrochloric acid) produces an easily soluble intermediate salt which crystallises in thin copper-red needles (Wilm, Hadow, 1889). It only contains a small amount of chlorine, and apparently corresponds to a compound 5PtK2Cy4+ PtK2Cy4Cl2+ 24H2O. Under the action of an excess of ammonia both these chlorine products are converted either completely or in part (according to Wilm ammonia does not act upon PtK2Cy4) into PtCy2,2NH3,i.e.a platino-ammonia compound (seefurther on). It is also necessary to pay attention to the fact that ruthenium and osmium—which, as we know, give higher forms of oxidation than platinum—are also able to combine with a larger proportion of potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a crystallinehydroruthenocyanic acid, RuH4(CN)6, which is soluble in water and alcohol, and corresponds with the salts M4Ru(CN)6. There are exactly similar osmic compounds—for example, K4Os(CN)6,3H2O. The latter is obtained in the form of colourless, sparingly-soluble regular tablets on evaporating the solution obtained from a fused mixture of potassium osmiochloride, K2OsCl6, and potassium cyanide. These osmic and ruthenic compounds fully correspond with potassium ferrocyanide, K4Fe(CN)6,3H2O, not only in their composition but also in their crystalline form and reactions, which again demonstrates the close analogy between iron, ruthenium, and osmium, which we have shown by giving these three elements a similar position (in the eighth group) in the periodic system. For rhodium and iridium only salts of the same type as the ferricyanides, M3RCy6, are known, and for palladium only of the type M2PdCy4, which are analogous to the platinum salts. In all these examples aconstancy of the typesof the double cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, gold. The double cyanides of iron, ruthenium, osmium have the type K4R(CN)6; of cobalt, rhodium, iridium, the type K3R(CN)6; of nickel, palladium, platinum the type K2R(CN)4and K2R(CN)5; and for copper, silver, gold there are known KR(CN)2, so that the presence of 4, 3, 2, and 1 atoms of potassium corresponds with the order of the elements in the periodic system. Those types which we have seen in the ferrocyanides and ferricyanides of iron repeat themselves in all the platinoid metals, and this naturally leads to the conclusion that the formation of similar so-called double salts is of exactly the same nature as that of the ordinary salts. If, in expressing the union of the elements in the oxygen salts, the existence of anaqueous residue(hydroxyl group) be admitted, in which the hydrogen is replaced by a metal, we have then only to apply this mode of expression to the double salts and the analogy will be obvious, if only we remember that Cl2, (CN)2, SO4, &c., are equivalent to O, as we see in RO, RCl2, RSO4, &c. They all = X2, and, therefore, in point of fact, wherever X (= Cl or OH, &c.) can be placed, there (Cl2H), (SO4H), &c., can also stand. And as Cl2H = Cl + HCl and SO4H = OH + SO3, &c., it follows that molecules HCl or SO3, or, in general, whole molecules—for instance, NH3, H2O, salts, &c., can annex themselves to a compound containing X. (This is an indirect consequence of the law of substitution which explains the origin of double salts, ammonia compounds, compounds with water of crystallisation, &c., by one general method.) Thus the double salt MgSO4,K2SO4, according to this reasoning,may beconsidered as a substance of the same type as MgCl2, namely, as = Mg(SO4K)2, and the alums as derived from Al(OH)(SO4), namely, as Al(SO4K)(SO4). Without stopping to pursue this digression further, we will apply these considerations to the type of the ferrocyanides and ferricyanides and their platinum analogues. Such a salt as K2PtCy4may accordingly be regarded as Pt(Cy2K)2, like Pt(OH)2; and such a salt as PtK2Cy5as PtCy(Cy2K)2, the analogue of PtX(OH)2, or AlX(OH)2, and other compounds of the type RX3. Potassium ferricyanide and the analogous compounds of cobalt, iridium, and rhodium, belong to the same type, with the same difference as there is between RX(OH)2and R(OH)3, since FeK3Cy6= Fe(Cy2K)3. Limiting myself to these considerations, which may partially elucidate the nature of double salts, I will now pass again to the complex saline compounds known for platinum.
(A) On mixing a solution of potassium thiocyanate with a solution of potassium platinosochloride, K2PtCl4, they form a double thiocyanate, PtK2(CNS)4, which is easily soluble in water and alcohol, crystallises in red prisms, and gives an orange-coloured solution, which precipitates salts of the heavy metals. The action of sulphuric acid on the lead salt of the same type gives the acid itself, PtH2(SCN)4, which corresponds with these salts. The type of these compounds is evidently the same as that of the cyanides.
(B)Platinous chloride, PtCl2, which is insoluble in water, formsdouble salts with the metallic chlorides. These double chlorides are soluble in water, and capable of crystallising. Hence when a hydrochloric acid solution of platinous chloride is mixed with solutions of metallic salts and evaporated it forms crystalline salts of a red or yellow colour. Thus, for example, the potassium salt, PtK2Cl4, is red, and easily soluble in water; the sodium salt is also soluble in alcohol; the barium salt, PtBaCl4,3H2O, is soluble in water, but the silver salt, PtAg2Cl4, is insoluble in water, and may be used for obtaining the remaining salts by means of double decomposition with their chlorides.
(C) A remarkable example of the complex compounds of platinum was observed by Schützenberger (1868). He showed that finely-divided platinum in the presence of chlorine and carbonic oxide at 250°-300° gives phosgene and a volatile compound containing platinum. The same substance is formed by the action of carbonic oxide on platinous chloride. It decomposes with an explosion in contact with water. Carbon tetrachloride dissolves a portion of this substance, and on evaporation gives crystals of 2PtCl2,3CO, whilst the compound PtCl2,2CO remains undissolved. When fused and sublimed it gives yellow needles of PtCl2,CO, and in the presence of an excess of carbonic oxide PtCl2,2CO is formed. These compounds are fusible (the first at 250°, the second at 142°, and the third at 195°). In this case (as in the double cyanides) combination takes place, because both carbonic oxide and platinous chloride are unsaturated compounds capable of further combination. The carbon tetrachloride solution absorbs NH3and gives PtCl2,CO,2NH3, and PtCl2,2CO,2NH3, and these substances are analogous (Foerster, Zeisel, Jörgensen) to similar compounds containing complex amines (for instance, pyridine, C5H5N), instead of NH3, and ethylene, &c., instead of CO, so that here we have a whole series of complex platino-compounds. The compound PtCl2CO dissolves in hydrochloric acid without change, and the solution disengages all the carbonic oxide when KCN is added to it, which shows that those forces which bind 2 molecules of KCN to PtCl2can also bind the molecule CO, or 2 molecules of CO. When the hydrochloric acid solution of PtCl2CO is mixed with a solution of sodium acetate or acetic acid, it gives a precipitate of PtOCO,i.e.the Cl2is replaced by oxygen (probably because the acetate is decomposed by water). This oxide, PtOCO, splits up into Pt + CO2at 350°. PtSCO is obtained by the action of sulphuretted hydrogen upon PtCl2CO. All this leads to the conclusion that the group PtCO is able to assimilate X2= Cl2, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by igniting spongy platinum at 250°, first in a stream of chlorine, and then in a stream of carbonic oxide, obtained (besides volatile products) a non-volatile yellow substance which remained unchanged in air and disengaged chlorine and phosgene gas when ignited; its composition was PtCl6(CO)2, which apparently proves it to be a compound of PtCl2and 2COCl2, as PtCl2is able to combine with oxychlorides, and forms somewhat stable compounds.
(D) The faculty of platinous chloride for forming stable compounds with divers substances shows itself in the formation of the compound PtCl2,PCl3by the action of phosphorus pentachloride at 250° on platinum powder (Pd reacts in a similar manner, according to Fink, 1892). The product contains both phosphorus pentachloride and platinum, whilst the presence of PtCl2is shown in the fact that the action of water produceschlorplatino-phosphorous acid, PtCl2P(OH)3.
(E) After the cyanides, thedouble saltsof platinumformed by sulphurous acidare most distinguished for their stability and characteristic properties. This is all the more instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as in all its compounds, it exhibits a dual reaction. The salts of sulphurous acid, R2SO3, either react as salts of a feeble bibasic acid, where the group SO3presents itself as bivalent, and consequently equal to X2, or else they react after the manner of salts of a monobasic acid containing the same residue, RSO3, as occurs in the salts of sulphuric acid. In sulphurous acid this residue is combined with hydrogen, H(SO3H), whilst in sulphuric acid it is united with the aqueous residue (hydroxyl), OH(SO3H). These two forms of action of the sulphites appear in their reactions with the platinum salts—that is to say, salts of both kinds are formed, and they both correspond with the type PtH2X4. The one series of salts contain PtH2(SO3)2, and their reactions are due to the bivalent residue of sulphurous acid, which replaces X2. The others, which have the composition PtR2(SO3H)4, contain sulphoxyl. The latter salts will evidently react like acids; they are formed simultaneously with the salts of the first kind, and pass into them. These salts are obtained either by directly dissolving platinous oxide in water containing sulphurous acid, or by passing sulphurous anhydride into a solution of platinous chloride in hydrochloric acid. If a solution of platinous chloride or platinous oxide in sulphurous acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate containing PtNa2(SO3Na)4,7H2O. If this precipitate be dissolved in a small quantity of hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of the other type, PtNa2(SO3)2,H2O, in the form of a yellow powder, which is sparingly soluble in water. The potassium salt analogous to the first salt, PtK2(SO3K)4,2H2O, is precipitated by passing sulphurous anhydride into a solution of potassium sulphite in which platinous oxide is suspended. A similar salt is known for ammonium, and with hydrochloric acid it gives a salt of the second kind, Pt(NH4)2(SO3)2,H2O. If ammonio-chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first deoxidised, and chlorine is evolved, forming a salt of the type PtX2; a double decomposition then takes place with the ammonium sulphite, and a salt of the composition Pt(NH4)2Cl3(SO3H) is formed (in a desiccator). The acid character of this substance is explained by the fact that it contains the elements SO3H—sulphoxyl, with the hydrogen not yet displaced by a metal. On saturating a solution of this acid with potassium carbonate it gives orange-coloured crystals of a potassium salt of the composition Pt(NH4)2Cl3(SO3K). Here it is evident that an equivalent of chlorine in Pt(NH4)2Cl4is replaced by the univalent residue of sulphurous acid. Among these salts, that of the composition Pt(NH4)Cl2(SO3H)2,H2O is very readily formed, and crystallises in well-formed colourless crystals; it is obtained by dissolving ammonium platinosochloride, Pt(NH4)2Cl4, in an aqueous solution of sulphurous acid. The difficulty with which sulphurous anhydride and platinum are separated from these salts indicates the same basic character in these compounds as is seen in the double cyanides of platinum. In their passage into a complex salt, the metal platinum and the group SO2modify their relations (compared with those of PtX2or SO2X2), just as the chlorine in the salts KClO, KClO3, and KClO4is modified in its relations as compared with hydrochloric acid or potassium chloride.
(F) No less characteristic are theplatinonitritesformed by platinous oxide. They correspond with nitrous acid, whose salts, RNO2, contain the univalent radicle, NO2, which is capable of replacing chlorine, and therefore the salts of this kind should form a common type PtR2(NO2)4, and such a salt of potassium has actually been obtained by mixing a solution of potassium platinosochloride with a solution of potassium nitrite, when the liquid becomes colourless, especially if it be heated, which indicates the change in the chemical distribution of the elements. As the liquid decolorises it gradually deposits sparingly soluble, colourless prisms of the potassium salt K2Pt(NO2)4, which does not contain any water. With silver nitrate a solution of this salt gives a precipitate of silver platinonitrite, PtAg2(NO2)4. The silver of this salt may be replaced by other metals by means of double decomposition with metallic chlorides. The sparingly soluble barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble acid, which separates, under the receiver of an air-pump, in red crystals; this acid has the composition PtH2(NO2)4. To the potassium salt, K2Pt(NO2)4, there correspond (Vèzes, 1892) K2Pt(NO2)4Br2and K2Pt(NO2)4Cl2and other compounds of the same type K2PtX6, where X is partly replaced by Cl or Br and partly by (NO2), showing a transition towards the type of the double salts like the platino-ammoniacal salts. (The corresponding double sodium nitrite salt of cobalt is soluble in water, while the K,NH4and many other salts are insoluble in water, as I was informed by Prof. K. Winkler in 1894).
In all the preceding complex compounds of Pt we see a common type PtX2,2MX (i.e.of double salts corresponding to PtO) or PtM2X4= Pt(MX2)2, corresponding to Pt(HO)2with the replacement of O by its equivalent X2. Two other facts must also be noted. In the first place these X's generally correspond to elements (like chlorine) or groups (like CN, NO2, SO3, &c.), which are capable of further combination. In the second place all the compounds of the type PtM2X4are capable of combining with chlorine or similar elements, and thus passing into compounds of the types PtX3or PtX4.
[12]The platinum salt and ammonia, when once combined together, are no longer subject to their ordinary reactions but form compounds which are comparatively very stable. The question at once suggests itself to all who are acquainted with these phenomena, as to what is the relation of the elements contained in these compounds. The first explanation is that these compounds are salts of ammonium in which the hydrogen is partially replaced by platinum. This is the view, with certain shades of difference, held by many respecting the platino-ammonium compounds. They were regarded in this light by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we suppose the hydrogen in 2NH4X to be replaced by bivalent platinum (as in the salts PtX2), we shall obtainNH3NH3PtXX—that is, the compound PtX2,2NH3. The compound with 4NH3will then be represented by a further substitution of the hydrogen in ammonia by ammonium itself—i.e.as NH2(NH4X)2Pt or PtX2,4NH3. A modification of this view is found in that representation of compounds of this kind which is based on atomicity. As platinum in PtX2is bivalent, has two affinities, and ammonia, NH3, is also bivalent, because nitrogen is quinquivalent and is here only combined with H3, it is evident what bonds should be represented in PtX2,2NH3and in PtX2,4NH3. In the former, Pt(NH3Cl)2, the nitrogen of each atom of ammonia is united by three affinities with H3, by one with platinum, and by the fifth with chlorine. The other compound is Pt(NH3.NH3Cl)2—that is, the N is united by one affinity with the other N, whilst the remaining bonds are the same as in the first salt. It is evident that this union or chain of ammonias has no obvious limit, and the most essential fault of such a mode of representation is that it does not indicate at all what number of ammonias are capable of being retained by platinum. Moreover, it is hardly possible to admit the bond between nitrogen and platinum in such stable compounds, for these kinds of affinities are, at all events, feeble, and cannot lead to stability, but would rather indicate explosive and easily-decomposed compounds. Moreover, it is not clear why this platinum, which is capable of giving PtX4, does not act with its remaining affinities when the addition of ammonia to PtX2takes place. These, and certain other considerations which indicate the imperfection of this representation of the structure of the platino-ammonium salts, cause many chemists to incline more to the representations of Berzelius, Claus, Gibbs, and others, who suppose that NH3is able to combine with substances, to adjoin itself or pair itself with them (this kind of combination is called ‘Paarung’) without altering the fundamental capacity of a substance for further combinations. Thus, in PtX2,2NH3, the ammonia is the associate of PtX2, as is expressed by the formula N2H6PtX2. Without enlarging on the exposition of the details of this doctrine, we will only mention that it, like the first, does not render it possible to foresee a limit to the compounds with ammonia; it isolates compounds of this kind into a special and artificial class; does not show the connection between compounds of this and of other kinds, and therefore it essentially only expresses the fact of the combination with ammonia and the modification in its ordinary reactions. For these reasons we do not hold to either of these proposed representations of the ammonio-platinum compounds, but regard them from the point of view cited above with reference to double salts and water of crystallisation—that is, we embrace all these compounds under the representation of compounds of complex types. The type of the compound PtX2,2NH3is far more probably the same as that of PtX2,2Z—i.e.as PtX4, or, still more accurately and truly, it is a compound of the same type as PtX2,2KX or PtX2,2H2O, &c. Although the platinum first entered into PtK2X4as the type PtX2, yet its character has changed in the same manner as the character of sulphur changes when from SO2the compound SO2(OH)2is obtained, or when KClO4, the higher form, is obtained from KCl. For us as yet there is no question as towhataffinities hold X2and what hold 2NH3, because this is a question which arises from the supposition of the existence of different affinities in the atoms, which there is no reason for taking as a common phenomenon. It seems to me that it is most importantas a commencementto render clear the analogy in the formation of various complex compounds, and it is this analogy of the ammonia compounds with those of water of crystallisation and double salts that forms the main object of the primary generalisation. We recognise in platinum, at all events, not only the four affinities expressed in the compound PtCl4, but a much larger number of them, if only thesummation of affinitiesis actually possible. Thus, in sulphur we recognise not two but a much greater number of affinities; it is clear that at least six affinities can act. So also among the analogues of platinum: osmic anhydride, OsO4, Ni(CO)4, PtH2Cl6, &c. indicate the existence of at least eight affinities; whilst, in chlorine, judging from the compound KClO4= ClO3(OK) = ClX7, we must recognise at least seven affinities, instead of the one which is accepted. The latter mode of calculating affinities is a tribute to that period of the development of science when only the simplest hydrogen compounds were considered, and when all complex compounds were entirely neglected (they were placed under the class of molecular compounds). This is insufficient for the present state of knowledge, because we find that, in complex compounds as in the most simple, the same constant types or modes of equilibrium are repeated, and the character of certain elements is greatly modified in the passage from the most simple into very complex compounds.Judging from the most complex platino-ammonium compounds PtCl4,4NH3, we should admit the possibility of the formation of compounds of the type PtX4Y4, where Y4= 4X2= 4NH3, and this shows that those forces which form such a characteristic series of double platinocyanides PtK2(CN)4,3H2O, probably also determine the formation of the higher ammonia derivatives, as is seen on comparing—PtCl2NH32Cl23NH3Pt(CN)2KCNKCN3H2O.Moreover, it is obviously much more natural to ascribe the faculty for combination withnY to the whole of the acting elements—that is, to PtX2or PtX4, and not to platinum alone. Naturally such compounds are not produced with any Y. With certain X's there only combine certain Y's. The best known and most frequently-formed compounds of this kind are those with water—that is, compounds with water of crystallisation. Compounds with salts are double salts; also we know that similar compounds are also frequently formed by means of ammonia. Salts of zinc, ZnX2, copper, CuX2, silver, AgX, and many others give similar compounds, but these and many otherammonio-metallicsaline compounds are unstable, and readily part with their combined ammonia, and it is only in the elements of the platinum group and in the group of the analogues of iron, that we observe the faculty to form stable ammonio-metallic compounds. It must be remembered that the metals of the platinum and iron groups are able to form several high grades of oxidation which have an acid character, and consequently in the lower degrees of combination there yet remain affinities capable of retaining other elements, and they probably retain ammonia, and hold it the more stably, because all the properties of the platinum compounds are rather acid than basic—that is, PtXnrecalls rather HX or SnXnor CXnthan KX, CaX2, BaX2, &c., and ammonia naturally will rather combine with an acid than with a basic substance. Further, a dependence, or certain connection of the forms of oxidation with the ammonia compounds, is seen on comparing the following compounds:PdCl2,2NH3,H2OPdCl2,4NH3,H2OPtCl2,2NH3PtCl4,4NH3RhCl3,5NH3RuCl2,4NH3,3H2OIrCl3,5NH3OsCl2,4NH3,2H2OWe know that platinum and palladium give compounds of lower types than iridium and rhodium, whilst ruthenium and osmium give the highest forms of oxidation; this shows itself in this case also. We have purposely cited the same compounds with 4NH3for osmium and ruthenium as we have for platinum and palladium, and it is then seen that Ru and Os are capable of retaining 2H2O and 3H2O, besides Cl2and NH3, which the compounds of platinum and palladium are unable to do. The same ideas which were developed in Note35, Chapter XXII. respecting the cobaltia compounds are perfectly applicable to the present case,i.e.to theplatiniacompounds or ammonia compounds of the platinum metals, among which Rh and Ir give compounds which are perfectly analogous to the cobaltia compounds.Iridium and rhodium, which easily give compounds of the type RX3, give compounds (Claus) of the type IrX3,5NH3, of a rose colour, and RhX3,5NH3, of a yellow colour. Jörgensen, in his researches on these compounds, showed their entire analogy with the cobalt compounds, as was to be expected from the periodic system.
[12]The platinum salt and ammonia, when once combined together, are no longer subject to their ordinary reactions but form compounds which are comparatively very stable. The question at once suggests itself to all who are acquainted with these phenomena, as to what is the relation of the elements contained in these compounds. The first explanation is that these compounds are salts of ammonium in which the hydrogen is partially replaced by platinum. This is the view, with certain shades of difference, held by many respecting the platino-ammonium compounds. They were regarded in this light by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we suppose the hydrogen in 2NH4X to be replaced by bivalent platinum (as in the salts PtX2), we shall obtainNH3NH3PtXX—that is, the compound PtX2,2NH3. The compound with 4NH3will then be represented by a further substitution of the hydrogen in ammonia by ammonium itself—i.e.as NH2(NH4X)2Pt or PtX2,4NH3. A modification of this view is found in that representation of compounds of this kind which is based on atomicity. As platinum in PtX2is bivalent, has two affinities, and ammonia, NH3, is also bivalent, because nitrogen is quinquivalent and is here only combined with H3, it is evident what bonds should be represented in PtX2,2NH3and in PtX2,4NH3. In the former, Pt(NH3Cl)2, the nitrogen of each atom of ammonia is united by three affinities with H3, by one with platinum, and by the fifth with chlorine. The other compound is Pt(NH3.NH3Cl)2—that is, the N is united by one affinity with the other N, whilst the remaining bonds are the same as in the first salt. It is evident that this union or chain of ammonias has no obvious limit, and the most essential fault of such a mode of representation is that it does not indicate at all what number of ammonias are capable of being retained by platinum. Moreover, it is hardly possible to admit the bond between nitrogen and platinum in such stable compounds, for these kinds of affinities are, at all events, feeble, and cannot lead to stability, but would rather indicate explosive and easily-decomposed compounds. Moreover, it is not clear why this platinum, which is capable of giving PtX4, does not act with its remaining affinities when the addition of ammonia to PtX2takes place. These, and certain other considerations which indicate the imperfection of this representation of the structure of the platino-ammonium salts, cause many chemists to incline more to the representations of Berzelius, Claus, Gibbs, and others, who suppose that NH3is able to combine with substances, to adjoin itself or pair itself with them (this kind of combination is called ‘Paarung’) without altering the fundamental capacity of a substance for further combinations. Thus, in PtX2,2NH3, the ammonia is the associate of PtX2, as is expressed by the formula N2H6PtX2. Without enlarging on the exposition of the details of this doctrine, we will only mention that it, like the first, does not render it possible to foresee a limit to the compounds with ammonia; it isolates compounds of this kind into a special and artificial class; does not show the connection between compounds of this and of other kinds, and therefore it essentially only expresses the fact of the combination with ammonia and the modification in its ordinary reactions. For these reasons we do not hold to either of these proposed representations of the ammonio-platinum compounds, but regard them from the point of view cited above with reference to double salts and water of crystallisation—that is, we embrace all these compounds under the representation of compounds of complex types. The type of the compound PtX2,2NH3is far more probably the same as that of PtX2,2Z—i.e.as PtX4, or, still more accurately and truly, it is a compound of the same type as PtX2,2KX or PtX2,2H2O, &c. Although the platinum first entered into PtK2X4as the type PtX2, yet its character has changed in the same manner as the character of sulphur changes when from SO2the compound SO2(OH)2is obtained, or when KClO4, the higher form, is obtained from KCl. For us as yet there is no question as towhataffinities hold X2and what hold 2NH3, because this is a question which arises from the supposition of the existence of different affinities in the atoms, which there is no reason for taking as a common phenomenon. It seems to me that it is most importantas a commencementto render clear the analogy in the formation of various complex compounds, and it is this analogy of the ammonia compounds with those of water of crystallisation and double salts that forms the main object of the primary generalisation. We recognise in platinum, at all events, not only the four affinities expressed in the compound PtCl4, but a much larger number of them, if only thesummation of affinitiesis actually possible. Thus, in sulphur we recognise not two but a much greater number of affinities; it is clear that at least six affinities can act. So also among the analogues of platinum: osmic anhydride, OsO4, Ni(CO)4, PtH2Cl6, &c. indicate the existence of at least eight affinities; whilst, in chlorine, judging from the compound KClO4= ClO3(OK) = ClX7, we must recognise at least seven affinities, instead of the one which is accepted. The latter mode of calculating affinities is a tribute to that period of the development of science when only the simplest hydrogen compounds were considered, and when all complex compounds were entirely neglected (they were placed under the class of molecular compounds). This is insufficient for the present state of knowledge, because we find that, in complex compounds as in the most simple, the same constant types or modes of equilibrium are repeated, and the character of certain elements is greatly modified in the passage from the most simple into very complex compounds.
Judging from the most complex platino-ammonium compounds PtCl4,4NH3, we should admit the possibility of the formation of compounds of the type PtX4Y4, where Y4= 4X2= 4NH3, and this shows that those forces which form such a characteristic series of double platinocyanides PtK2(CN)4,3H2O, probably also determine the formation of the higher ammonia derivatives, as is seen on comparing—
Moreover, it is obviously much more natural to ascribe the faculty for combination withnY to the whole of the acting elements—that is, to PtX2or PtX4, and not to platinum alone. Naturally such compounds are not produced with any Y. With certain X's there only combine certain Y's. The best known and most frequently-formed compounds of this kind are those with water—that is, compounds with water of crystallisation. Compounds with salts are double salts; also we know that similar compounds are also frequently formed by means of ammonia. Salts of zinc, ZnX2, copper, CuX2, silver, AgX, and many others give similar compounds, but these and many otherammonio-metallicsaline compounds are unstable, and readily part with their combined ammonia, and it is only in the elements of the platinum group and in the group of the analogues of iron, that we observe the faculty to form stable ammonio-metallic compounds. It must be remembered that the metals of the platinum and iron groups are able to form several high grades of oxidation which have an acid character, and consequently in the lower degrees of combination there yet remain affinities capable of retaining other elements, and they probably retain ammonia, and hold it the more stably, because all the properties of the platinum compounds are rather acid than basic—that is, PtXnrecalls rather HX or SnXnor CXnthan KX, CaX2, BaX2, &c., and ammonia naturally will rather combine with an acid than with a basic substance. Further, a dependence, or certain connection of the forms of oxidation with the ammonia compounds, is seen on comparing the following compounds:
We know that platinum and palladium give compounds of lower types than iridium and rhodium, whilst ruthenium and osmium give the highest forms of oxidation; this shows itself in this case also. We have purposely cited the same compounds with 4NH3for osmium and ruthenium as we have for platinum and palladium, and it is then seen that Ru and Os are capable of retaining 2H2O and 3H2O, besides Cl2and NH3, which the compounds of platinum and palladium are unable to do. The same ideas which were developed in Note35, Chapter XXII. respecting the cobaltia compounds are perfectly applicable to the present case,i.e.to theplatiniacompounds or ammonia compounds of the platinum metals, among which Rh and Ir give compounds which are perfectly analogous to the cobaltia compounds.
Iridium and rhodium, which easily give compounds of the type RX3, give compounds (Claus) of the type IrX3,5NH3, of a rose colour, and RhX3,5NH3, of a yellow colour. Jörgensen, in his researches on these compounds, showed their entire analogy with the cobalt compounds, as was to be expected from the periodic system.
[13]Subsequently, a whole series of such compounds was obtained with various elements in the place of the (non-reacting) chlorine, and nevertheless they, like the chlorine, reacted with difficulty, whilst the second portion of the X's introduced into such salts easily underwent reaction. This formed the most important reason for the interest which the study of the composition and structure of the platino-ammonium salts subsequently presented to many chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt, Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and others. The salts PtX4,2NH3, discovered by Gerhardt, also exhibited several different properties in the two pairs of X's. In the remaining platino-ammonium salts all the X's appear to react alike.The quality of the X's, retainable in the platino-ammonium salts, may be considerably modified, and they may frequently be wholly or partially replaced by hydroxyl. For example, the action of ammonia on the nitrate of Gerhardt's base, Pt(NO3)4,2NH3, in a boiling solution, gradually produces a yellow crystalline precipitate which is nothing else than abasic hydrateoralkali, Pt(OH)4,2NH3. It is sparingly soluble in water, but gives directly soluble salts PtX4,2NH3with acids. The stability of this hydroxide is such that potash does not expel ammonia from it, even on boiling, and it does not change below 130°. Similar properties are shown by the hydroxide Pt(OH)2,2NH3and the oxide PtO,2NH3of Reiset's second base. But the hydroxides of the compounds containing 4NH3are particularly remarkable. The presence of ammonia renders them soluble and energetic. The brevity of this work does not permit us, however, to mention many interesting particulars in connection with this subject.
[13]Subsequently, a whole series of such compounds was obtained with various elements in the place of the (non-reacting) chlorine, and nevertheless they, like the chlorine, reacted with difficulty, whilst the second portion of the X's introduced into such salts easily underwent reaction. This formed the most important reason for the interest which the study of the composition and structure of the platino-ammonium salts subsequently presented to many chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt, Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and others. The salts PtX4,2NH3, discovered by Gerhardt, also exhibited several different properties in the two pairs of X's. In the remaining platino-ammonium salts all the X's appear to react alike.
The quality of the X's, retainable in the platino-ammonium salts, may be considerably modified, and they may frequently be wholly or partially replaced by hydroxyl. For example, the action of ammonia on the nitrate of Gerhardt's base, Pt(NO3)4,2NH3, in a boiling solution, gradually produces a yellow crystalline precipitate which is nothing else than abasic hydrateoralkali, Pt(OH)4,2NH3. It is sparingly soluble in water, but gives directly soluble salts PtX4,2NH3with acids. The stability of this hydroxide is such that potash does not expel ammonia from it, even on boiling, and it does not change below 130°. Similar properties are shown by the hydroxide Pt(OH)2,2NH3and the oxide PtO,2NH3of Reiset's second base. But the hydroxides of the compounds containing 4NH3are particularly remarkable. The presence of ammonia renders them soluble and energetic. The brevity of this work does not permit us, however, to mention many interesting particulars in connection with this subject.
[14]Hydroxides are known corresponding with Gros's salts, which contain one hydroxyl group in the place of that chlorine or haloid which in Gros's salts reacts with difficulty, and these hydroxides do not at once show the properties of alkalis, just as the chlorine which stands in the same place does not react distinctly; but still, after the prolonged action of acids, this hydroxyl group is also replaced by acids. Thus, for example, the action of nitric acid on Pt(NO3)2Cl2,4NH3causes the non-active chlorine to react, but in the product all the chlorine is not replaced by NO3, but only half, and the other half is replaced by the hydroxyl group: Pt(NO3)2Cl2,4NH3+ HNO3+ H2O = Pt(NO3)3(OH),4NH3+ 2HCl; and this is particularly characteristic, because here the hydroxyl group has not reacted with the acid—an evident sign of the non-alkaline character of this residue. I think it may be well to call attention to the fact that the composition of the ammonio-metallosalts very often exhibits a correspondence between the amount of X's and the amount of NH3, of such a nature that we find they contain either XNH3or the grouping X2NH3; for example, Pt(XNH3)2and Pt(X2NH3)2, Co(X2NH3)3, Pt(XNH3)4, &c. Judging from this, the view of the constitution of the double cyanides of platinum given in Note11finds some confirmation here, but, in my opinion, all questions respecting the composition (and structure) of the ammoniacal, double, complex, and crystallisation compounds stand connected with the solution of questions respecting the formation of compounds of various degrees of stability, among which a theory of solutions must be included, and therefore I think that the time has not yet come for a complete generalisation of the data which exist for these compounds; and here I again refer the reader to Prof. Kournakoff's work cited in Chapter XXII., Note35. However, we may add a few individual remarks concerning the platinia compounds.To the common properties of the platino-ammonium salts, we must add not only theirstability(feeble acids and alkalis do not decompose them, the ammonia is not evolved by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed in them to as great an extent as those of iron in the ferricyanides. Thus neither alkalis nor hydrogen sulphide will separate the platinum from them. For example, sulphuretted hydrogen in acting on Gros's salts gives sulphur, removes half the chlorine by means of its hydrogen, and forms salts of Reiset's first base. This may be understood or explained by considering the platinum in the molecule as covered, walled up by the ammonia, or situated in the centre of the molecule, and therefore inaccessible to reagents. On this assumption, however, we should expect to find clearly-expressed ammoniacal properties, and this is not the case. Thus ammonia is easily decomposed by chlorine, whilst in acting on the platino-ammonium salts containing PtX2and 2NH3or 4NH3, chlorine combines and does not destroy the ammonia; it converts Reiset's salts into those of Gros and Gerhardt. Thus from PtX2,2NH3there is formed PtX2Cl2,2NH3, and from PtX2,4NH3the salt of Gros's base PtX2Cl2,4NH3. This shows that the amount of chlorine which combines is not dependent on the amount of ammonia present, but is due to the basic properties of platinum. Owing to this some chemists suppose the ammonia to be inactive or passive in certain compounds. It appears to me that these relations, these modifications, in the usual properties of ammonia and platinum are explained directly by their mutual combination. Sulphur, in sulphurous anhydride, SO2, and hydrogen sulphide, SH2, is naturally one and the same, but if we only knew of it in the form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride, we should consider its properties as hidden. The oxygen in magnesia, MgO, and in nitric peroxide, NO2, is so different that there is no resemblance. Arsenic no longer reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and in their compounds with nitrogen all metals modify both their reactions and their physical properties. We are accustomed to judge the metals by their saline compounds with haloid groups, and ammonia by its compounds with acid substances, and here, in the platino-compounds, if we assume the platinum to be bound to the entire mass of the ammonia—to its hydrogen and nitrogen—we shall understand that both the platinum and ammonia modify their characters. Far more complicated is the question why a portion of the chlorine (and other haloid simple and complex groups) in Gros's salts acts in a different manner from the other portion, and why only half of it acts in the usual way. But this also is not an exclusive case. The chlorine in potassium chlorate or in carbon tetrachloride does not react with the same ease with metals as the chlorine in the salts corresponding with hydrochloric acid. In this case it is united to oxygen and carbon, whilst in the platino-ammonium compounds it is united partly to platinum and partly to the platino-ammonium group. Many chemists, moreover, suppose that a part of the chlorine is united directly to the platinum and the other part to the nitrogen of the ammonia, and thus explain the difference of the reactions; but chlorine united to platinum reacts as well with a silver salt as the chlorine of ammonium chloride, NH4Cl, or nitrosyl chloride, NOCl, although there is no doubt that in this case there is a union between the chlorine and nitrogen. Hence it is necessary to explain the absence of a facile reactive capacity in a portion of the chlorine by the conjoint influence of the platinum and ammonia on it, whilst the other portion may be admitted as being under the influence of the platinum only, and therefore as reacting as in other salts. By admitting a certain kind of stable union in the platino-ammonium grouping, it is possible to imagine that the chlorine does not react with its customary facility, because access to a portion of the atoms of chlorine in this complex grouping is difficult, and the chlorine union is not the same as we usually meet in the saline compounds of chlorine. These are the grounds on which we, in refuting the now accepted explanations of the reactions and formation of the platino-compounds, pronounce the following opinion as to their structure.In characterising the platino-ammonium compounds, it is necessary to bear in mind that compounds which already contain PtX4do not combine directly with NH3, and that such compounds as PtX4,4NH3only proceed from PtX2, and therefore it is natural to conclude that those affinities and forces which cause PtX2to combine with X2also cause it to combine with 2NH3. And having the compound PtX2,2NH3, and supposing that in subsequently combining with Cl2it reacts with those affinities which produce the compounds of platinic chloride, PtCl4, with water, potassium chloride, potassium cyanide, hydrochloric acid, and the like, we explain not only the fact of combination, but also many of the reactions occurring in the transition of one kind of platino-ammonium salts into another. Thus by this means we explain the fact that (1) PtX2,2NH3combines with 2NH3, forming salts of Reiset's first base; (2) and the fact that this compound (represented as follows for distinctness), PtX2,2NH3,2NH3, when heated, or even when boiled in solution, again passes into PtX2,2NH3(which resembles the easy disengagement of water of crystallisation, &c.); (3) the fact that PtX2,2NH3is capable of absorbing, under the action of the same forces, a molecule of chlorine, PtX2,2NH3,Cl2, which it then retains with energy, because it is attracted, not only by the platinum, but also by the hydrogen of the ammonia; (4) the fact that this chlorine held in this compound (of Gerhardt) will have a position unusual in salts, which will explain a certain (although very feebly-marked) difficulty of reaction; (5) the fact that this does not exhaust the faculty of platinum for further combination (we need only recall the compound PtCl4,2HCl,16H2O), and that therefore both PtX2,2NH3,Cl2and PtX2,2NH3,2NH3are still capable of combination, whence the latter, with chlorine, gives PtX2,2NH3,2NH3,Cl2, after the type of PtX4Y4(and perhaps higher); (6) the fact that Gros's compounds thus formed are readily reconverted into the salts of Reiset's first base when acted on by reducing agents; (7) the fact that in Gros's salts, PtX2,2NH3(NH3X)2, the newly-attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is attached both to the platinum and to the ammonia, for both of which it has an attraction; (8) the fact that the faculty for further combination is not even yet exhausted in the type of Gros's salts, and that we actually have a compound of Gros's chlorine salt with platinous chloride and with platinic chloride; the salt PtSO4,2NH3,2NH3,SO4combines further also with H2O; (9) the fact that such a faculty of combination with new molecules is naturally more developed in the lower forms of combination than in the higher. Hence the salts of Reiset's first base—for example, PtCl2,2NH3,2NH3—both combine with water and give precipitates (soluble in water but not in hydrochloric acid) of double salts with many salts of the heavy metals—for example, with lead chloride, cupric chloride, and also with platinic and platinous chlorides (Buckton's salts). The latter compounds will have the composition PtCl2,2NH3,2NH3,PtCl2—that is, the same composition as the salts of Reiset's second base, but it cannot be identical with it. Such an interesting case does actually exist. The first salt, PtCl2,4NH3,PtCl2, is green, insoluble in water and in hydrochloric acid, and is known asMagnus's salt, and the second, PtCl2,2NH3, is Reiset's yellow, sparingly soluble (in water). They are polymeric, namely, the first contains twice the number of elements held in the second, and at the same time they easily pass into each other. If ammonia be added to a hot hydrochloric acid solution of platinous chloride, it forms the salt PtCl2,4NH3, but in the presence of an excess of platinous chloride it gives Magnus's salt. On boiling the latter in ammonia it gives a colourless soluble salt of Reiset's first base, PtCl2,4NH3, and if this be boiled with water, ammonia is disengaged, and a salt of Reiset's second base, PtCl2,2NH3, is obtained.A class of platino-ammonium isomerides (obtained by Millon and Thomsen) are also known. Buckton's salts—for example, the copper salt—were obtained by them from the salts of Reiset's first base, PtCl2,4NH3, by treatment with a solution of cupric chloride, &c., and therefore, according to our method of expression, Buckton's copper salt will be PtCl2,4NH3,CuCl2. This salt is soluble in water, but not in hydrochloric acid. In it the ammonia must be considered as united to the platinum. But if cupric chloride be dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is added to it, a violet precipitate is obtained of the same composition as Buckton's salt, which, however, is insoluble in water, but soluble in hydrochloric acid. In this a portion, if not all, of the ammonia must be regarded as united to the copper, and it must therefore be represented as CuCl2,4NH3,PtCl2. This form is identical in composition but different in properties (is isomeric) with the preceding salt (Buckton's). The salt of Magnus is intermediate between them, PtCl2,4NH3,PtCl2; it is insoluble in water and hydrochloric acid. These and certain other instances of isomeric compounds in the series of the platino-ammonium salts throw a light on the nature of the compounds in question, just as the study of the isomerides of the carbon compounds has served and still serves as the chief cause of the rapid progress of organic chemistry. In conclusion, we may add that (according to the law of substitution) we must necessarily expect all kinds of intermediate compounds between the platino and analogous ammonia derivatives on the one hand, and the complex compounds of nitrous acid on the other. Perhaps the instance of the reaction of ammonia upon osmic anhydride, OsO4, observed by Fritsche, Frémy, and others, and more fully studied by Joly (1891), belongs to this class. The latter showed that when ammonia acts upon an alkaline solution of OsO4the reaction proceeds according to the equation: OsO4+ KHO + NH3= OsNKO3+ 2H2O. It might be imagined that in this case the ammonia is oxidised, probably forming the residue of nitrous acid (NO), while the type OsO4is deoxidised into OsO2, and a salt, OsO(NO)(KO), of the type OsX4is formed. This salt crystallises well in light yellow octahedra. It corresponds toosmiamic acid, OsO(ON)(HO), whose anhydride [OsO(NO)]2, has the composition Os2N2O5, which equals 2Os + N2O5to the same extent as the above-mentioned compound PtCO2equals Pt + CO2(seeNote11).
[14]Hydroxides are known corresponding with Gros's salts, which contain one hydroxyl group in the place of that chlorine or haloid which in Gros's salts reacts with difficulty, and these hydroxides do not at once show the properties of alkalis, just as the chlorine which stands in the same place does not react distinctly; but still, after the prolonged action of acids, this hydroxyl group is also replaced by acids. Thus, for example, the action of nitric acid on Pt(NO3)2Cl2,4NH3causes the non-active chlorine to react, but in the product all the chlorine is not replaced by NO3, but only half, and the other half is replaced by the hydroxyl group: Pt(NO3)2Cl2,4NH3+ HNO3+ H2O = Pt(NO3)3(OH),4NH3+ 2HCl; and this is particularly characteristic, because here the hydroxyl group has not reacted with the acid—an evident sign of the non-alkaline character of this residue. I think it may be well to call attention to the fact that the composition of the ammonio-metallosalts very often exhibits a correspondence between the amount of X's and the amount of NH3, of such a nature that we find they contain either XNH3or the grouping X2NH3; for example, Pt(XNH3)2and Pt(X2NH3)2, Co(X2NH3)3, Pt(XNH3)4, &c. Judging from this, the view of the constitution of the double cyanides of platinum given in Note11finds some confirmation here, but, in my opinion, all questions respecting the composition (and structure) of the ammoniacal, double, complex, and crystallisation compounds stand connected with the solution of questions respecting the formation of compounds of various degrees of stability, among which a theory of solutions must be included, and therefore I think that the time has not yet come for a complete generalisation of the data which exist for these compounds; and here I again refer the reader to Prof. Kournakoff's work cited in Chapter XXII., Note35. However, we may add a few individual remarks concerning the platinia compounds.
To the common properties of the platino-ammonium salts, we must add not only theirstability(feeble acids and alkalis do not decompose them, the ammonia is not evolved by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed in them to as great an extent as those of iron in the ferricyanides. Thus neither alkalis nor hydrogen sulphide will separate the platinum from them. For example, sulphuretted hydrogen in acting on Gros's salts gives sulphur, removes half the chlorine by means of its hydrogen, and forms salts of Reiset's first base. This may be understood or explained by considering the platinum in the molecule as covered, walled up by the ammonia, or situated in the centre of the molecule, and therefore inaccessible to reagents. On this assumption, however, we should expect to find clearly-expressed ammoniacal properties, and this is not the case. Thus ammonia is easily decomposed by chlorine, whilst in acting on the platino-ammonium salts containing PtX2and 2NH3or 4NH3, chlorine combines and does not destroy the ammonia; it converts Reiset's salts into those of Gros and Gerhardt. Thus from PtX2,2NH3there is formed PtX2Cl2,2NH3, and from PtX2,4NH3the salt of Gros's base PtX2Cl2,4NH3. This shows that the amount of chlorine which combines is not dependent on the amount of ammonia present, but is due to the basic properties of platinum. Owing to this some chemists suppose the ammonia to be inactive or passive in certain compounds. It appears to me that these relations, these modifications, in the usual properties of ammonia and platinum are explained directly by their mutual combination. Sulphur, in sulphurous anhydride, SO2, and hydrogen sulphide, SH2, is naturally one and the same, but if we only knew of it in the form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride, we should consider its properties as hidden. The oxygen in magnesia, MgO, and in nitric peroxide, NO2, is so different that there is no resemblance. Arsenic no longer reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and in their compounds with nitrogen all metals modify both their reactions and their physical properties. We are accustomed to judge the metals by their saline compounds with haloid groups, and ammonia by its compounds with acid substances, and here, in the platino-compounds, if we assume the platinum to be bound to the entire mass of the ammonia—to its hydrogen and nitrogen—we shall understand that both the platinum and ammonia modify their characters. Far more complicated is the question why a portion of the chlorine (and other haloid simple and complex groups) in Gros's salts acts in a different manner from the other portion, and why only half of it acts in the usual way. But this also is not an exclusive case. The chlorine in potassium chlorate or in carbon tetrachloride does not react with the same ease with metals as the chlorine in the salts corresponding with hydrochloric acid. In this case it is united to oxygen and carbon, whilst in the platino-ammonium compounds it is united partly to platinum and partly to the platino-ammonium group. Many chemists, moreover, suppose that a part of the chlorine is united directly to the platinum and the other part to the nitrogen of the ammonia, and thus explain the difference of the reactions; but chlorine united to platinum reacts as well with a silver salt as the chlorine of ammonium chloride, NH4Cl, or nitrosyl chloride, NOCl, although there is no doubt that in this case there is a union between the chlorine and nitrogen. Hence it is necessary to explain the absence of a facile reactive capacity in a portion of the chlorine by the conjoint influence of the platinum and ammonia on it, whilst the other portion may be admitted as being under the influence of the platinum only, and therefore as reacting as in other salts. By admitting a certain kind of stable union in the platino-ammonium grouping, it is possible to imagine that the chlorine does not react with its customary facility, because access to a portion of the atoms of chlorine in this complex grouping is difficult, and the chlorine union is not the same as we usually meet in the saline compounds of chlorine. These are the grounds on which we, in refuting the now accepted explanations of the reactions and formation of the platino-compounds, pronounce the following opinion as to their structure.
In characterising the platino-ammonium compounds, it is necessary to bear in mind that compounds which already contain PtX4do not combine directly with NH3, and that such compounds as PtX4,4NH3only proceed from PtX2, and therefore it is natural to conclude that those affinities and forces which cause PtX2to combine with X2also cause it to combine with 2NH3. And having the compound PtX2,2NH3, and supposing that in subsequently combining with Cl2it reacts with those affinities which produce the compounds of platinic chloride, PtCl4, with water, potassium chloride, potassium cyanide, hydrochloric acid, and the like, we explain not only the fact of combination, but also many of the reactions occurring in the transition of one kind of platino-ammonium salts into another. Thus by this means we explain the fact that (1) PtX2,2NH3combines with 2NH3, forming salts of Reiset's first base; (2) and the fact that this compound (represented as follows for distinctness), PtX2,2NH3,2NH3, when heated, or even when boiled in solution, again passes into PtX2,2NH3(which resembles the easy disengagement of water of crystallisation, &c.); (3) the fact that PtX2,2NH3is capable of absorbing, under the action of the same forces, a molecule of chlorine, PtX2,2NH3,Cl2, which it then retains with energy, because it is attracted, not only by the platinum, but also by the hydrogen of the ammonia; (4) the fact that this chlorine held in this compound (of Gerhardt) will have a position unusual in salts, which will explain a certain (although very feebly-marked) difficulty of reaction; (5) the fact that this does not exhaust the faculty of platinum for further combination (we need only recall the compound PtCl4,2HCl,16H2O), and that therefore both PtX2,2NH3,Cl2and PtX2,2NH3,2NH3are still capable of combination, whence the latter, with chlorine, gives PtX2,2NH3,2NH3,Cl2, after the type of PtX4Y4(and perhaps higher); (6) the fact that Gros's compounds thus formed are readily reconverted into the salts of Reiset's first base when acted on by reducing agents; (7) the fact that in Gros's salts, PtX2,2NH3(NH3X)2, the newly-attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is attached both to the platinum and to the ammonia, for both of which it has an attraction; (8) the fact that the faculty for further combination is not even yet exhausted in the type of Gros's salts, and that we actually have a compound of Gros's chlorine salt with platinous chloride and with platinic chloride; the salt PtSO4,2NH3,2NH3,SO4combines further also with H2O; (9) the fact that such a faculty of combination with new molecules is naturally more developed in the lower forms of combination than in the higher. Hence the salts of Reiset's first base—for example, PtCl2,2NH3,2NH3—both combine with water and give precipitates (soluble in water but not in hydrochloric acid) of double salts with many salts of the heavy metals—for example, with lead chloride, cupric chloride, and also with platinic and platinous chlorides (Buckton's salts). The latter compounds will have the composition PtCl2,2NH3,2NH3,PtCl2—that is, the same composition as the salts of Reiset's second base, but it cannot be identical with it. Such an interesting case does actually exist. The first salt, PtCl2,4NH3,PtCl2, is green, insoluble in water and in hydrochloric acid, and is known asMagnus's salt, and the second, PtCl2,2NH3, is Reiset's yellow, sparingly soluble (in water). They are polymeric, namely, the first contains twice the number of elements held in the second, and at the same time they easily pass into each other. If ammonia be added to a hot hydrochloric acid solution of platinous chloride, it forms the salt PtCl2,4NH3, but in the presence of an excess of platinous chloride it gives Magnus's salt. On boiling the latter in ammonia it gives a colourless soluble salt of Reiset's first base, PtCl2,4NH3, and if this be boiled with water, ammonia is disengaged, and a salt of Reiset's second base, PtCl2,2NH3, is obtained.
A class of platino-ammonium isomerides (obtained by Millon and Thomsen) are also known. Buckton's salts—for example, the copper salt—were obtained by them from the salts of Reiset's first base, PtCl2,4NH3, by treatment with a solution of cupric chloride, &c., and therefore, according to our method of expression, Buckton's copper salt will be PtCl2,4NH3,CuCl2. This salt is soluble in water, but not in hydrochloric acid. In it the ammonia must be considered as united to the platinum. But if cupric chloride be dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is added to it, a violet precipitate is obtained of the same composition as Buckton's salt, which, however, is insoluble in water, but soluble in hydrochloric acid. In this a portion, if not all, of the ammonia must be regarded as united to the copper, and it must therefore be represented as CuCl2,4NH3,PtCl2. This form is identical in composition but different in properties (is isomeric) with the preceding salt (Buckton's). The salt of Magnus is intermediate between them, PtCl2,4NH3,PtCl2; it is insoluble in water and hydrochloric acid. These and certain other instances of isomeric compounds in the series of the platino-ammonium salts throw a light on the nature of the compounds in question, just as the study of the isomerides of the carbon compounds has served and still serves as the chief cause of the rapid progress of organic chemistry. In conclusion, we may add that (according to the law of substitution) we must necessarily expect all kinds of intermediate compounds between the platino and analogous ammonia derivatives on the one hand, and the complex compounds of nitrous acid on the other. Perhaps the instance of the reaction of ammonia upon osmic anhydride, OsO4, observed by Fritsche, Frémy, and others, and more fully studied by Joly (1891), belongs to this class. The latter showed that when ammonia acts upon an alkaline solution of OsO4the reaction proceeds according to the equation: OsO4+ KHO + NH3= OsNKO3+ 2H2O. It might be imagined that in this case the ammonia is oxidised, probably forming the residue of nitrous acid (NO), while the type OsO4is deoxidised into OsO2, and a salt, OsO(NO)(KO), of the type OsX4is formed. This salt crystallises well in light yellow octahedra. It corresponds toosmiamic acid, OsO(ON)(HO), whose anhydride [OsO(NO)]2, has the composition Os2N2O5, which equals 2Os + N2O5to the same extent as the above-mentioned compound PtCO2equals Pt + CO2(seeNote11).