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Cobalt(II) bromide

Cobalt(II) bromide refers to inorganic compounds with the formula . The anhydrous form (n = 0) is a green solid and the hexahydrate (n = 6) is a red solid. These compounds find some use as catalysts.

Structure

The anhydrous compound has a cadmium iodide structure. The tetrahydrate is molecular, with the formula trans-[CoBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>].

Preparation and reactions

Cobalt(II) bromide form by treating an aqueous suspension of cobalt(II) carbonate with hydrobromic acid according to the following idealized equation:

Anhydrous cobalt(II) bromide is hygroscopic. Air exposure eventually forms the hexahydrate in air, which appears as red-purple crystals. The hexahydrate loses four water of crystallization molecules at 100&nbsp;°C forming the dihydrate:

CoBr<sub>2</sub>·6H<sub>2</sub>O → CoBr<sub>2</sub>·2H<sub>2</sub>O + 4 H<sub>2</sub>O

The anhydrous compound forms by heating any of the hydrates to >150 °C in a vacuum:

The resulting solid can be purified by vacuum sublimation at > 500 °C.

Further heating to 130&nbsp;°C produces the anhydrous form:

CoBr<sub>2</sub>·2H<sub>2</sub>O → CoBr<sub>2</sub> + 2 H<sub>2</sub>O

At higher temperatures, cobalt(II) bromide reacts with oxygen, forming cobalt(II,III) oxide and bromine vapor.

The coordination compound bromopentaamminecobalt(III) bromide is prepared by oxidation of an aqueous solution of cobalt(II) bromide and ammonia.

2 CoBr<sub>2</sub> + 8 NH<sub>3</sub> + 2 NH<sub>4</sub>Br + H<sub>2</sub>O<sub>2</sub> → 2 [Co(NH<sub>3</sub>)<sub>5</sub>Br]Br<sub>2</sub> + 2 H<sub>2</sub>O

Triphenylphosphine complexes of cobalt(II) bromide have been used as a catalysts in organic synthesis.

Safety

Exposure to large amounts of cobalt(II) can cause cobalt poisoning. Bromide is also mildly toxic.

References