Vanadous Bromide, VBr₃

Vanadous Bromide, vanadium tribromide, VBr₃, is conveniently obtained synthetically. When powdered vanadium is gently warmed with pure, dry bromine, combination takes place readily, considerable heat is evolved and the tribromide is formed. Ferrovanadium alloy gives the same product. It is also obtained by the action of pure, dry bromine vapour on either vanadium nitride, VN, or a mixture of vanadous oxide, V₂O₃, and charcoal, or on pure vanadium carbide, V₄C₃. It is a dark green or greyish-black, amorphous, deliquescent substance, which decomposes spontaneously with evolution of bromine at ordinary temperatures, giving a dark liquid. It dissolves in water without evolution of bromine to give a green solution, which possesses the same general properties as a solution of vanadium trichloride. On being gently heated in air it is rapidly converted into a mixture of vanadous oxide, V₂O₃, and vanadium pentoxide, V₂O₅.

The hexahydrate, VBr₃.6H₂O, is prepared by dissolving the anhydrous tribromide in air-free water and concentrating, first on a water-bath and subsequently in vacuo. It can also be prepared by dissolving vanadous hydroxide, V(OH)₃, in concentrated hydrobromic acid out of contact with air, or by the electrolytic reduction of a solution of vanadium pentoxide, V₂O₅, in hydrobromic acid. Hydrated vanadium tribromide is a green, hygroscopic, crystalline powder, which is less easily crystallisable and less stable than the corresponding chloride. The colour of its solution in water varies from brown to yellow according to the concentration; addition of acid gives rise to a green coloration which changes to blue as oxidation to the tetravalent state ensues.

Hexammino-vanadium Tribromide, [V(NH₃)₆]Br₃, is prepared by the action of liquid ammonia on vanadium tribromide. Its properties and reactions are similar to those of the corresponding chlorine compound.