Note 12 — Chapter XXIII · THE PLATINUM METALS

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 obtain NH3 NH3 Pt X X —that is, the compound PtX2,2NH3. The compound with 4NH3 will 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 PtX2 is 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 PtX2,2NH3 and 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 PtX2 takes 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 NH3 is 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 PtX2,2NH3 is 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 PtK2X4 as the type PtX2, yet its character has changed in the same manner as the character of sulphur changes when from SO2 the compound SO2(OH)2 is obtained, or when KClO4, the higher form, is obtained from KCl. For us as yet there is no question as to what affinities hold X2 and 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 important as a commencement to 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 the summation of affinities is 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—

PtCl₂ NH₃ 2Cl₂ 3NH₃

Pt(CN)₂ KCN KCN 3H₂O.

Moreover, it is obviously much more natural to ascribe the faculty for combination with nY to the whole of the acting elements—that is, to PtX₂ or 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 other ammonio-metallic saline 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, PtX_n recalls rather HX or SnX_n or CX_n than 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,H₂O PdCl₂,4NH₃,H₂O

PtCl₂,2NH₃ PtCl₄,4NH₃

RhCl₃,5NH₃ RuCl₂,4NH₃,3H₂O

IrCl₃,5NH₃ OsCl₂,4NH₃,2H₂O

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 4NH₃ for 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 Cl₂ and NH₃, which the compounds of platinum and palladium are unable to do. The same ideas which were developed in Note 35, Chapter XXII. respecting the cobaltia compounds are perfectly applicable to the present case, i.e. to the platinia compounds 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.