Platinocyanides

According to some authorities, the first metal ammine to be isolated in the solid state was the reddish yellow hexaamminecobalt(III) oxalate [Co(NH3)6]2(C204)3, described in 1822 by Leopold Gmelin (1788-1853).16 Gmelin also discovered several new double salts potassium ferricyanide, or potassium hexacyanoferrate(III), K3[Fe(CN)6l>17 the cobalticyanides or hexacyanocobaltates(III), M3[Co(CN) ],18 and the platinocyanides or tetracyanoplatinates(II), M2[Pt(CN)4].19All these substances certainly deserve to rank among the earliest known coordination compounds. 

K[(CH3)3Pt]Pt(CN)4, is isolated from an aqueous solution of the nitrate by adding potassium platinocyanide. A slimy yellow precipitate is formed, insoluble in water and the usual organic solvents except acetone and alcohol. On evaporation from one of these solvents an amorphous resinous greenish-yellow compound is produced which, from determination of the platinum, appears to correspond to K2Pt(CN)4[(CH3)3Pt]2Pt(CN)4. 

A soluble form of platinous cyanide is obtained as a yellow precipitate by decomposing a solution of a platinocyanide with sulphuric acid. It dissolves easily in alkalies and in potassium cyanide solution, but is usually contaminated with small quantities of sulphate. 

By double decomposition of the metallic sulphate with barium platinocyanide solution. Thus, for example, the magnesium salt was first prepared by Martius as follows ... 

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 ... 

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 ... 

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.3... 

Isomerism.—In an interesting series of memoirs, Levj 3 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 tetra-hydrate 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 ... 

Levy, Studies on Platinocyanides, Doctorate Thesis, London University, 1912 Trans. Chem. Soc., 1906, 89, 125 1908, 93, 1446 1912, ioi, 1081 Proc. Comb. Phil. 

Fluorescence.—The majority of platinocyanides fluoresce under the stimulus of ultra-violet light1 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. 

Oxidation.—In early literature 2 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(CNJs. 3H20, 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 3 (in the case of bromine)... 

Oxidation of potassium platinocyanide with lead peroxide, acidulated with dilute sulphuric acid, yields a product containing the S04 group,1 of formula3 a K2Pt(CN)4.K2Pt(CN)4.S04, 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 8K2Pt(CN)4.KPt(CN)4.6H20. Lithium platinocyanide is more readily oxidised in this manner, yielding 2Li2Pt(CN) 4. LiPt(CN)1 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.5... 

Aluminium Platinocyanide, Al2[Pt(CN)4]3, is prepared by addition of an aqueous solution of aluminium sulphate to one of barium platinocyanide. The barium sulphate is removed by filtration and the clear liquid allowed to crystallise. The salt separates out in blue, deliquescent laminse.3... 

Ammonium Platinocyanide, (NH4)2Pt(CN)4.2H20, is obtained when freshly prepared platinous cyanide is dissolved in ammonium cyanide solution.4 This method, however, docs not give a perfectly pure product. 

Barium platinocyanide tetrahydrate can exist in two isomeric forms, which, however, are crystallographically identical. They differ in colour, density, and in fluorescence, as indicated in the following table 2 ... 

When a solution of barium platinocyanide is allowed to crystallise after adding a trace of barium cyanide or barium hydroxide to it, crystals of the green 8-salt axe obtained. If, on the other hand, a trace of hydrogen cyanide, hydrogen chloride, tartaric or acetic acid, or cyanoplatinous acid is present instead, crystals of the yellow a-salt are deposited. 

Barium platinocyanide, after prolonged exposure to radiation from radium compounds, becomes brown or red and loses its fluorescent properties. The only way to completely restqje the red crystals to their fluorescent state is by solution and recrystallisation,1 although gentle heating will partially restore them. This is well illustrated by the following table ... 

Calcium Platinocyanide, CaPt(CN)4.5H20, may be prepared in a similar manner to the barium salt.3 It crystallises in rhombic prisms, having the following crystallographic elements 4 ... 

Levy5 has shown that two isomeric varieties of the pentahydrate exist, analogous to the two isomerides of the tetrahydrate of barium platinocyanide. The a-salt may be described as golden yellow with green fluorescence on the pyramid faces, whilst the (3-salt shows a green fluorescence on all faces, which is particularly intense on the tops of the crystals. The crystalline characters of the two isomerides are identical. 

The a-salt is obtained by crystallisation from a solution containing a small quantity of acid, and the (3-salt from one containing some hydroxyl or cyanide ions, precisely as in the case of the a and [3 barium platinocyanides. 

Anhydrous calcium platinocyanide, CaPt(CN)4, is white, and exhibits only a feeble fluorescence. 

Calcium Ammonium and Calcium Potassium Platinocyanides have also been obtained.3... 

Cerium Platinocyanide,6 Cej[Pt(CN)4]3,18H20, yields fluorescent crystals in two varieties, namely, yellow a-crystals and greenish yellow (3-crystals, analogous to the two calcium isomerides.5... 

Indium Platinocyanide, In2[Pt(CN4)]3.2H20, is prepared3 by boiling solutions of barium platinocyanide and indium sulphate together, filtering off the barium sulphate and allowing to crystallise out. It yields white, hygroscopic leaflets. 

Lead Platinocyanide, PbPt(CN)4, is readily prepared by mixing boiling saturated solutions of potassium platinocyanide and lead acetate. The solution remains clear, but on cooling the lead salt separates out as a yellowish white crystalline powder.4 It is anhydrous, and becomes yellow on warming in air, then white whilst at higher temperatures cyanogen is liberated, the residue containing metallic lead and platinum. 

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