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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.
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