Tellurium Tetrabromide, TeBr₄

Tellurium Tetrabromide, TeBr₄ can be produced by the action of excess of bromine on tellurium, the vigour of the reaction being moderated by cooling. The excess of bromine is subsequently removed by evaporation. There is a danger of the product containing a little of the dibromide or a little unaltered tellurium; it may therefore be purified by fractional sublimation under reduced

pressure, the tetrabromide being less volatile than the dibromide and more volatile than any oxybromide which may be present.

The same direct combination of the elements can be effected under an aqueous solution of hydrobromic acid, the resulting tetrabromide being obtained by evaporation of the solution.

Tellurium dioxide may be converted into the tetrabromide by means of hydrobromic acid or a mixture of potassium bromide and phosphoric acid in aqueous solution.

The reddish-yellow solid can be obtained in the form of prismatic needles by slow sublimation. It has a density of 4.31 and melts near 380° C. to a deep red liquid. Ebullition, which occurs near 420° C., is not effected under ordinary conditions of pressure without partial dissociation of the vapour into dibromide and bromine. The boiling-point is therefore not constant.

Tellurium tetrabromide is slightly hygroscopic and can be dissolved in a little water without decomposition. The aqueous solution, on evaporation over sulphuric acid, yields a deep red hydrated product. More water causes hydrolysis to tellurous and hydrobromic acids:

TeBr₄ + 3H₂O = H₂TeO₈ + 4HBr.

A solution of the tetrabromide in aqueous 50 per cent, tartaric acid solution becomes colourless on dilution, and the tellurium remains as tartrate.

Silver cyanide acts on a benzene solution of the tetrabromide, replacing bromine by the cyanogen radical, with simultaneous reduction, the product being a solid tellurium dicyanide:

TeBr₄ + 3AgCN = Te(CN)₂ + 3AgBr + CNBr.

With ammonia the tetrabromide reacts similarly to the tetrachloride, and also like the latter, it forms additive compounds with the halides of the alkali metals and ammonium and with organic bases. The telluribromides formed are deep red, crystalline solids, of the general formula X₂TeBr₆; they dissolve without decomposition in aqueous solutions of hydrobromic acid, but are decomposed more or less readily by water. The additive compound with hydrogen bromide, TeBr₄. HBr.5H₂O, is slightly more stable than the chlorine analogue.