Potassium Octacyanomolybdate IV 2-Hydrate

University of Illinois, Urbana, 111., unpublished observations. (A closer investigation revealed more lines in the powder pattern than could be accounted for by the cubic structure.) 

Various methods for the preparation of potassium octacyano-molybdate(IV) 2-hydrate from molybdenum(VI) oxide, molyb-dic acid, or molybdates have been reported. These methods, although in general rehable, have one unfavorable aspect in common low efficiencies due to conversions of less than 50%. 

This can be explained by the fact that these syntheses all consist of two separate steps. In the first step the molybdenum is reduced in acid medium from the +6 to the +5 oxidation state. In the second step potassium cyanide is introduced, and formation of the molybdenum (IV) cyanide complex takes place in alkaline medium as a result of disproportionation of the molybdenum (V) into the +4 and -1-6 oxidation states. As a result of the disproportionation, the conversion cannot possibly exceed 50%. One method described in the literature is not subject to this limitation however, the procedure is rather lengthy and complex.  

The synthesis described here is simpler and more efficient because the reduction and complexation are accomplished at the same time in a solution containing Mo (VI) and cyanide. Hydrazine, which is known to reduce Mo (VI) to Mo(V) in acid medium, is used as the reducing agent. The reaction proceeds on the slow, drop-by-drop addition of concentrated hydrochloric acid in an amount insufficient to make the overall reaction mixture acidic. It appears that the reduction reaction takes place primarily in the locally acid regions that result from this mode of addition. 

The properties of potassium octacyanomolybdate(IV) 2-hydrate have been described in reference 1, where an oxidimetric method of analysis is described. The purity of the compound may be conveniently determined spectrophotometrically, using the absorption band at 367.7 m/z (emax = 170).  

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Potassium nitridorhenate, 6 167 Potassium nitridotrisulfate, 2 182 Potassium nitrocarbamate, potassium salt, 1 68, 70 Potassium V-nitrosohydroxylamine-V-sulfonate, 5 117 by metathesis, 6 120 Potassium octacyanomolybdate(IV) 2-hydrate, 3 160 Potassium osmiamate, 6 204... 

On heating at 110°, the potassium octacyanomolyb-date(IV) 2-hydrate loses its water of hydration. The salt is extremely soluble in water, from which it may be crystallized in the form of yellow rhombic bipyramidal crystals, (a b c = 0.7028 1 0.3711). It is oxidized by cerium(IV) salt solution to the corresponding octacyanomolybdate(V), K3Mo(CN)g. Aqueous solutions are photosensitive and, on prolonged exposure to direct sunlight, turn red and then pale green with the evolution of hydrogen cyanide. 

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