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.