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Nickel Cyanide, Ni(CN)2

When potassium cyanide is added to a solution of a nickel salt, a green precipitate of Nickel Cyanide, Ni(CN)2, is produced. This readily dissolves in excess of the reagent, yielding potassium nickelo-cyanide, K2Ni(CN)4. Solutions of this salt contain the complex ion Ni(CN)4, so that the above formula is probably more correct than 2KCN.Ni(CN)2. It crystallises with one molecule of water, K2Ni(CN)4.H2O. Density 1.875 at 11°C., and 1.871 at 14.5° C. It is readily decomposed by the addition of sodium hypobromite, hydrated nickel peroxide being thrown down as a black precipitate. The corresponding cobalt salt is, in like circumstances, oxidised to the stable cobalti-cyanide which remains in solution. This fact is made use of in separating nickel and cobalt in qualitative analysis. Nickel does not yield stable salts corresponding to the ferro- and ferri-cyanides.

The tetrahydrate, Ni(CN)2.4H2O, separates after prolonged standing as steel-blue leaflets from a cold, aqueous solution of nickel sulphate, potassium cyanide, and ammonia.

On prolonged standing the ammoniacal solution of nickel cyanide deposits crystalline needles of the ammoniate, 2Ni(CN)2.2NH3.H2O. This, at 250° C., is converted into the anhydrous cyanide.
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