Yellow Salts

The acid from which Yellow Salts are derived, molybdocyanic acid, H₄Mo(CN)₈.6H₂O, has been prepared by adding hydrochloric acid, of density 1.19, to a concentrated solution of the potassium salt (see below), extracting the precipitate with alcohol, and precipitating with ether in a freezing mixture. It forms yellow needles, readily soluble in water and alcohol, the solutions being stable and yielding on neutralisation with bases the corresponding salts.

The potassium salt, K₄Mo(CN)₈.2H₂O, first obtained by Chilesotti, as already described, yields yellow rhombic-bipyramidal crystals:

a:b:c = 0.7028:1:0.3711.

When heated to 110° C. it loses its water of crystallisation. It is readily soluble in water.

The cadmium salt, Cd₂Mo(CN)₈.8H₂O, forms light yellow microscopic needles, insoluble in water, soluble in ammonia.

The manganese salt, Mn₂Mo(CN)₈.8H₂O, yields bright yellow glistening leaflets.

The thallium salt, Tl₄Mo(CN)₈, occurs in long, lustrous, reddish- yellow needles, slightly soluble in water.

Metal-ammine salts have also been obtained by Rosenheim and his co-workers by dissolving the molybdicyanides in boiling concentrated ammonia and allowing to crystallise: the cadmium-ammine, Cd₂(NH₃)₄Mo(CN)₈.2H₂O, deep yellow needles; the copper-ammine, Cu₂(NH₃)₄Mo(CN)₈.2H₂O, deep green needles; the nickel-ammine, [Ni(NH₃)₄]₂Mo(CN)₈.8H₂O, bluish-grey needles; and the silver-ammine, Ag₄(NH₃)₃Mo(CN)₈, lustrous yellow needles. It is probable that the last-named salt has 4 molecules of NH₃ when first formed.