Ammonium Hexabromoosmate IV

Submitted by P. P. Dwyer and J. W. Hogarth Checked by Richard N. RhodaI 

Potassium and ammonium hexahaloosmates(IV) are commonly used as starting materials for the preparation of various other osmium compounds. Osmium(VIII) oxide is the most convenient source of osmium for the preparation of these hexahalo complexes. 

Caution. Osmium(VIII) oxide is extremely toxic. The yellow solid melts at 31° and has an appreciable vapor pressure even at room temperature. The compound is often liberated when solutions of osmium compounds are treated with oxidizing agents. Ample precautions to protect eyes, nose, and mouth are essential. 

ampoule of osmium(VIII) oxide (ca. 0.004 mol) is broken into a 100-ml. Erlenmeyer flask, and 36 ml. of 47% hydrobromic acid is added immediately. The mixture is boiled under reflux for 2 hours. The solution is decanted from the portions of broken glass into a 125-ml. beaker, and the flask is rinsed with an additional 7 ml. of the hydrobromic acid solution. To the hot solution in the beaker, 3 g. of ammonium bromide (0.03 mol) is added. After the solid has dissolved completely, the mixture is cooled to room temperature, and 50 ml. of absolute ethanol is added with stirring. The dense black solid which forms is allowed to settle f and is then washed by decantation with absolute ethanol until the washings are nearly free from bromide ion. The residue is transferred to a sintered-glass filter and washed on the filter with absolute ethanol until the filtrate gives no turbidity with ethanolic silver nitrate solution. The product is then dried at 140°. 

Ammonium hexabromoosmate(IV) crystallizes as minute black cubes and octahedra. The compound dissolves sparingly in cold water to give a red solution. Hot aqueous solutions deposit black osmium(IV) oxide. The compound is soluble in warm glycerol or ethylene glycol but is insoluble in ethanol or methanol. 

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