Nickel Dibromide, NiBr₂

Nickelous Bromide or Nickel Dibromide, NiBr₂, may be obtained in the anhydrous condition by heating the finely divided metal in bromine vapour and subliming the product at bright red heat in the absence of air. A mixture of nitrogen and hydrogen bromide is particularly suitable. It may also be produced by addition of the calculated quantity of dry bromine to finely divided nickel in ether. A yellow salt of composition corresponding to the formula NiBr₂.(C₂H₅)₂O results, and this on heating yields the ether-free, anhydrous bromide. The salt usually occurs as golden scales which absorb moisture on exposure to air. The colour, however, varies according to the state of aggregation of the salt from yellow to dark bronze brown. At red heat in the presence of traces of air or moisture some nickelous oxide is formed, but Richards and Cushman could find no evidence of the existence of an oxybromide under such conditions.

The sublimed salt is slowly soluble in hot water, yielding a clear solution, which may be boiled without decomposition. Berthelot states that the solution on standing in air deposits nickel monoxide. This, however, is not the case with the pure substance. The density of the sublimed salt is 4.64 at 28° C.

The trihydrate, NiBr₂.3H₂O, may be obtained by introducing finely divided nickel into bromine under water, or by dissolving nickel monoxide in aqueous hydrogen bromide. On concentrating the solution, the salt crystallises out in the form of deliquescent needles.

The solubility of nickel bromide in water is as follows:

Temperature ° С	0	10	20	40	60	100
Grams NiBr2 in 100 grams solution.	53.0	55.0	56.7	59.1	60.4	60.8

The hexahydrate, NiBr₂.6H₂O, has been prepared. It melts at 28.5° C., and from the liquid the trihydrate crystallises out. The nonahydrate, NiBr₂.9H₂O, melting at -2.5° C. without decomposition, has also been isolated.

When ammonia is added to a solution of nickel bromide, beautiful violet crystals of the hexammoniate, NiBr₂.6NH₃, separate out. These are soluble in concentrated hot ammonium hydroxide, but insoluble in the cold. On boiling with excess of water, nickel dihydroxide is produced. Since cobalt does not yield a similar derivative, the formation of nickel hexammoniate forms a useful method of separating nickel from cobalt.