Potassium Vanadicyanide, K₃[V(CN)₆]

Potassium Vanadicyanide, K₃[V(CN)₆], is prepared by the addition of excess of concentrated potassium cyanide solution to a concentrated solution of vanadous chloride, VCl₃; precipitation in the cold with alcohol gives rise to small rhombohedral plates. The solution is not very stable and rapidly becomes turbid, while addition of an acid produces the green colour which is characteristic of the V^••• ion. The complex ion [V(CN)₆]''' appears, therefore, to be unstable, unlike the corresponding [Fe(CN)₆]''', [Cr(CN)₆]''', and [Co(CN)₆]''' complex ions. The solution reacts with salts of heavy metals to yield variously coloured precipitates of double cyanides.

An unstable double salt of potassium pyrovanadate and potassium cyanide, K₄V₂O₇.4KCN.14H₂O, has been prepared, as well as a complex addition compound of sodium vanadyl cyanide and hexamethylenetetramine, Na₃VO(CN)₅.2C₆H₁₂N₄.5H₂O.