Ammonium Pentabromooxomolybdate

A sample of 2 g. of ammonium para-molybdate is treated with 20 ml. of 9 M hydrobromic acid and is gently boiled to drive off the liberated bromine. The solution is evaporated almost to dryness. The process is repeated with a second 20-ml. portion of hydrobromic acid. Finally the residue is dissolved in 10 ml. of 9 M hydrobromic acid. The solution is concentrated to half volume and cooled in a freezing mixture. Anhydrous hydrogen bromide gas is rapidly passed through this solution to precipitate reddish-brown, crystalline ammonium pentabromooxomolybdate(2 —). This solid is filtered and dried in a vacuum desiccator over solid KOH to constant weight yield is 3 g. Anal. Calcd. for (NH4)2[MoOBr5] Mo, 17.52 Br, 73.00 N, 5.11. Found Mo, 18.01 Br, 72.51 N, 5.48. The oxidation state of the metal was found to be 4.99, and the magnetic moment pen = 1.79 B.M. at 303°K. 

The compound dissolves readily in water, and the aqueous solution reacts acidic to litmus owing to hydrolysis. Brown MoO(OH)3 is precipitated from this solution on adding ammonia 

Palmer, Experimental Inorganic Chemistry, p. 418, Cambridge University Press, New York, 1954. 

Submitted by GEORGE B. KAUFFMAN, RUSSELL FULLER, JAMES FELSER, CHARLES M. FLYNN, JR.,t and MICHAEL T. POPEt Checked by LOUIS C. W. BAKERt 

At a pH near 6, manganese(II) metavanadate precipitates and does not react further, whereas at a pH near 2, other heteropoly species are formed. Inasmuch as commercial potassium metavanadate is not suitable for the syntheses of the 13-vanadomanganate(IV) and nickelate(IV), directions for the preparation of this compound are included in the syntheses below, which are modified from the method of Flynn and Pope.5 The preparation of the yellow-tinged green reduced form of potassium 13-vanadomanganate(IV) is also included.5 

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