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Platinocyanides, M2Pt(CN)4

Cyanoplatinous acid, like chlorplatinous acid, yields a series of well- defined salts of general formula M2Pt(CN)4. By analogy they should be termed cyanoplatinites, but it is more usual to refer to them as platinocyanides. The salts definitely contain the negative radicle Pt(CN)2'', and are not, therefore, mere double salts of the type 2MCN.Pt(CN)2. They may be obtained by neutralising the free acid with their corresponding bases, but the method is not convenient. It is more usual to prepare them:
  1. By double decomposition of the metallic sulphate with barium platinocyanide solution. Thus, for example, the magnesium salt was first prepared by Martius as follows:

    MgSO4 + BaPt(CN)4 = MgPt(CN)4 + BaSO4.
  2. By precipitation of the metallic platinocyanide, when insoluble in water, by double decomposition of a soluble salt of the metal with potassium platinocyanide solution. In this manner Martius obtained the copper salt:

    CuSO4 + K2Pt(CN)4 = CuPt(CN)4 + K2SO4.
  3. By warming platinum salt solutions with alkali or alkaline earth cyanides and allowing to crystallise. In this manner the potassium salt, the first platinocyanide to be obtained, namely, by Ittner, was prepared:

    6KCN + PtCl4 = K2Pt(CN)4 + 4KCl + (CN)2.
  4. A more recent method consists in passing an alternating electric current through platinum electrodes immersed in aqueous solutions of the cyanides of the alkali or alkaline earth metals.


The platinocyanides are an interesting set of salts, remarkable, amongst other of their properties, for the tenacity with which they retain the electronegative metal (platinum) and disguise it to ordinary tests. Concentrated nitric or hydrochloric acid, alone or mixed, in the cold or on boiling, extracts no platinum from them. Even concentrated sulphuric acid only liberates platinum cyanide with difficulty.

The platinocyanides yield very beautiful hydrates. The magnesium salt is an excellent example. It yields, upon slow evaporation of its solution, large regular prisms which appear deep red by transmitted light, but viewed by reflected light the sides of the prisms exhibit a brilliant beetle-green, and the extremities a deep blue or purple. This is the heptahydrate, MgPt(CN)4.7H2O. When gently warmed, even under water, the crystals become yellow, which colour characterises the dihydrated crystals deposited from solution at 45° to 50° C.

Isomerism of Platinocyanides

In an interesting series of memoirs, Levy has shown that certain hydrated platinocyanides, notably those of barium and calcium, exist in two modifications, having the same chemical composition and crystalline form, but exhibiting a remarkable difference in their optical characters, differing in colour and in the intensity of their fluorescence. The barium and calcium salts show this most distinctly, and the cerium salt only to a small extent. Other platinocyanides do not show this at all.

It is not easy to account for this isomerism according to the ordinary methods of representation of the structure of the platinocyanides. Since samples of anhydrous barium platinocyanide, derived from the isomeric tetrahydrates respectively, are equally fluorescent, it seems fairly certain that any scheme must take into consideration the combined water. Levy therefore suggests that the isomerism of the tetrahydrate may be accounted for by assuming that two molecules of water are attached to the barium and two to the platinocyanide radicles, since, in the case of the latter radicles, two alternative positions are possible, namely:

and

It is conceivable that the (3 or asymmetrical form may be incapable of that movement under the stimulus of the incident radiations which, in the case of the a or symmetrical isomeride, are able to excite fluorescence.

Fluorescence of Platinocyanides

The majority of platinocyanides fluoresce under the stimulus of ultra-violet light or of radium radiations, although some salts show no sign of this property. Magnesium, erbium, yttrium, thallium and uranyl salts are cases in point.

It is certainly very remarkable that uranyl platinocyanide should fail to respond to the stimulus of ultra-violet light or to radium radiations. Both the uranyl and the platinocyanide groups can confer the property of fluorescence upon salts containing them, yet when they both occur in the same complex, there is no sign of fluorescence.

Fluorescence under the stimulus of radium emanations is usually exhibited in a marked degree by one hydrate of the platinocyanide whilst other hydrates either exhibit a feeble fluorescence or none at all. The anhydrous is frequently non-fluorescent also. For example, the heptahydrate of magnesium platinocyanide, MgPt(CN)4.7H2O, is not fluorescent, although the pentahydrate, MgPt(CN)4.5H2O, is markedly so. Again, the pentahydrate of calcium platinocyanide, CaPt(CN)4. 5H2O, exhibits intense fluorescence, whilst the anhydrous salt is only feebly fluorescent.

Oxidation of Platinocyanides

In early literature it is stated that by the action of chlorine, bromine, iodine, lead peroxide or nitric acid upon potassium platinocyanide, a new product, potassium platinicyanide, K2Pt(CN)5.3H2O, is obtained. Similar statements may be found in some recent text-books, but are nevertheless quite incorrect.

When potassium platinocyanide in aqueous solution is acted upon by chlorine or bromine, a halogenated product of composition corresponding to the formula (in the case of bromine)

K2Pt(CN)4.Br2.6K2Pt(CN)4.16H2O

is obtained, but no platinicyanide.

Oxidation of potassium platinocyanide with lead peroxide, acidulated with dilute sulphuric acid, yields a product containing the SO4 group, of formula xK2Pt(CN)4.K2Pt(CN)4.SO4, where x is probably 9; but again no platinicyanide is formed.

If, however, a solution of potassium platinocyanide is treated with hydrogen peroxide (20-volume solution) acidulated with dilute sulphuric acid, potassium platinicyanide, KPt(CN)4, is obtained, united with three molecules of platinocyanide, thus 3K2Pt(CN)4.KPt(CN)4.6H2O. Lithium platinocyanide is more readily oxidised in this manner, yielding 2Li2Pt(CN)4.LiPt(CN)4, and if the hydrogen peroxide is replaced by Merck's pure perhydrol, complete oxidation to lithium platinicyanide, LiPt(CN)4, is obtained.

The platinocyanides of the rare earth metals are crystalline salts which may be divided into two classes.
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