Tellurium Dichloride, TeCl₂

Tellurium Dichloride, TeCl₂ is the first product of the regulated action of chlorine on tellurium, but it is difficult to prevent further conversion into the tetrachloride. A more satisfactory procedure is to boil tellurium tetrachloride with tellurium under reflux:

TeCl₄ + Te = 2TeCl₂.

When powdered tellurium is heated in a stream of carbonyl chloride, brown vapours of the dichloride are evolved, which may be condensed to a velvety-black crystalline mass.

When in a state of fine division, tellurium dichloride has a yellowish- green colour. On account of its miscibility with tellurium the chloride is difficult to obtain pure and the recorded melting-points range from 209° to 175° C.; the boiling-point is near 324° C. The reddish vapour at 450° C. has a density in keeping with the unimolecular formula TeCl₂.

Bivalent tellurium is not stable and the dichloride in the solid state tends to undergo self-oxidation and reduction, forming a solid solution of the element in the tetrachloride. Under the action of water, acids or alkalis, tellurium and tellurous acid are formed:

2TeCl₂ + 3H₂O = Te + H₂TeO₃ + 4HCl.

The dichloride is stable only in the gaseous condition or in solution when in equilibrium with its decomposition products. It is hygroscopic, but does not fume in air. When heated in air it burns to tellurium dioxide and tetrachloride. It is also slowly converted by chlorine into the tetrachloride.

Repeated sublimation of tellurium dichloride with a deficiency of ammonium chloride leads to the formation of a greenish-black chlorotellurite, (NH₄)₂TeCl₄, which is stable in air and not hygroscopic.

The dichloride reacts with magnesium phenyl bromide, C₆H₅.MgBr, in ether solution, with formation of diphenyl telluride, (C₆H₅)₂Te. Tellurium dibromide and di-iodide give a similar result. In ether solution the dichloride absorbs bromine and iodine, but the products, presumably the chlorobromide and chloro-iodide, respectively, have not been isolated.