Antimony Tribromide, SbBr₃

Antimony Tribromide, SbBr₃, is formed spontaneously, and with incandescence, by the direct union of powdered antimony and bromine. For its preparation, bromine is placed in a retort, and excess of powdered antimony is added gradually, the mixture being kept cool until all the antimony has been added, after which the resulting antimony tribromide is distilled. It has also been prepared by the action of excess of antimony upon a solution of bromine in carbon disulphide. It may be purified by crystallisation from solution in the same solvent.

Antimony tribromide forms colourless, acicular rhombic crystals

a:b:c = 0.817:1:0.869

Its density is 4.148; it melts at 96.6° C. and boils, between 270° and 280° C,

The molecular volume, calculated from the density at -194° C., is 79.9.

The dipole moment (in solutions with organic solvents) is 2.47×10^-18 e.s.u.

Antimony tribromide absorbs moisture from the air, and is immediately decomposed by it with the formation of an oxybromide. The formation of this oxybromide is prevented by addition of tartaric acid. The tribromide is not attacked by either nitric acid or sulphuric acid in the cold, but on warming bromine is liberated in each case. It is partially soluble in ether, forming two liquid layers, the lower of which contains a compound of the two substances. It is soluble in boron tribromide. Nitric oxide is without action, but with nitrogen tetroxide a compound, 2Sb₂O₅.N₂O₅, is formed. The heat of formation is given by

Sb (solid) + 3Br (gas) = SbBr₃ (solid) + 76,900 gram-calories

A complex acid, H₂Sb₃Br₁₁, has been prepared by the action of hydrochloric acid on the salt K₂Sb₃Br₁₁ (see below). It is also obtained by the action of hydro- bromic acid on a solution of antimony tribromide in acetone. It forms yellow crystals. Corresponding to this acid numerous salts of the alkali metals, the alkaline earth metals and cadmium have been prepared from solutions of the mixed bromides in acetone. To these has been ascribed the general formula M₂Sb₃Br₁₁, where M = K, Na, Li, NH₄(2H₂O), and M₂ = Ba(2H₂O), Cd. Many of these salts are decomposed by water. From an examination of the electrolysis of their solutions evidence has been obtained of the existence of the complex anion Sb₃Br₁₁⁼ which subsequently dissociates into 3SbBr₃ and Br₂. The compounds K₂Sb₃Br₉F₂, K₂Sb₃Br₉I₂, SrSb₃Br₉Cl₂, and HgSb₆Br₁₈I₂ have also been obtained.

By substituting potassium nitrate, potassium thiocyanate or sodium azide for the alkali halide, the compounds K₂Sb₃Br₉(NO₃)₂, K₂Sb₃Br₉(CNS)₂ and NaSb₃Br₉N₃ have been obtained. The last is a white, crystalline substance.

In a similar manner the ammonium compound, (NH₄)₂SbBr₅.2H₂O, is obtained in the form of hexagonal prisms.

A vanadium compound, SbBr₃.VBr₄.7H₂O, has been obtained in the form of black, pointed prisms. It is hygroscopic and is decomposed by water.

A complex gold compound, 5RbBr.SbBr₃.2AuBr₃, has been prepared. It is a black substance, soluble in water and in hydrobromic acid. It contains no univalent gold.

Complex alkyl compounds have been obtained by treatment of antimony tribromide with mixtures of alkyl halides and alkyl sulphides. They appear to have the constitution [R₃S]SbBr₄.