Tellurium Tetrachloride, TeCl₄

When chlorine is passed continually over carefully heated tellurium, the final product is a yellow liquid of Tellurium Tetrachloride, TeCl₄, which solidifies to a crystalline mass on cooling. It can be purified by distillation. The chlorine can be replaced by sulphur monochloride, or even by sulphuryl chloride or thionyl chloride:

Te + 2Cl₂ = TeCl₄;
Te + 2S₂Cl₂ = TeCl₄ + 4S;
Te + 2SO₂Cl₂ = TeCl₄ + 2SO₂;
Te + 2SOCl₂ = TeCl₄ + SO₂ + S.

Tellurium dioxide is also converted into the tetrachloride by the action of sulphur monochloride or even hydrogen chloride.

Tellurium tetrachloride is a colourless, crystalline solid; it melts at 225° C. to a yellow liquid, the colour of which deepens as the boiling- point, 390° C., is approached. The vapour is yellow and at 440° C. has a density agreeing closely with the formula TeCl₄, although above 500° C. appreciable dissociation occurs, probably into the dichloride and chlorine. Unlike tellurium dichloride, the vapour of which shows marked absorption bands, the vapour of the tetrachloride gives no definite absorption spectrum.

The molten tetrachloride resembles the dichloride in readily conducting the electric current, tellurium in both cases being liberated at the cathode.

Although stable in dry air, the tetrachloride is gradually decomposed by moisture; it deliquesces, giving an oxychloride. This "oxychloride" is probably only a mixture of tetrachloride and dioxide. With a larger quantity of water the tetrachloride dissolves, giving rise finally to tellurium dioxide. It is soluble without decomposition in aqueous hydrochloric acid (from which an unstable compound HCl.TeCl₄.5H₂O crystallises at -30° C.), also in carbon disulphide and sulphur monochloride.

Ammonia attacks the tetrachloride differently at various temperatures. At 200° to 250° C. reduction is effected according to the equation:

3TeCl₄ + l6NH₃ = 3Te + 12NH₄Cl + 2N₂.

At 0° C. ammonia is absorbed, with formation of additive compounds; TeCl₄.6NH₃, TeCl₄.4NH₃ and TeCl₄.3NH₃ have been described. These compounds easily liberate ammonia on gently warming, and at higher temperatures form the lower chloride and ammonium chloride. Liquid ammonia (anhydrous) at still lower temperatures, e.g. -15° C., converts tellurium tetrachloride into the yellow, amorphous, explosive tellurium nitride and ammonium chloride.

With sulphur trioxide, tellurium tetrachloride yields a crystalline compound having the composition TeCl₄.2SO₃. With the chlorides of the alkali metals (including ammonium) and the organic amines, yellow tellurichlorides of the general formula X₂TeCl₂ are produced, corresponding with the unknown acid H₂TeCl₂. These are decomposed by water with formation of tellurous acid, but can generally be recrystallised from aqueous hydrochloric acid without decomposition.

When fused in a sealed tube with silver chloride the liquid separates into two layers, the lower consisting of almost pure silver chloride, whilst the upper contains AgCl—TeCl₄ eutectic with excess of AgCl. The eutectic melts at 210° C. and contains about 9 molecules per cent, of silver chloride. Metallic silver reacts with tellurium tetrachloride to form silver telluride and chloride, excess of the tetrachloride having no further action on silver telluride; thus in its halogen compounds tellurium behaves like a " noble " metal.

Additive compounds can be produced with aluminium chloride, phosphorus pentachloride and ethyl ether, the products being TeCl₄.2AlCl₃, TeCl₄.PCl₅ and TeCl₄.(C₂H₅)₂O, respectively.

Tellurium oxychloride is probably non-existent as a definite substance, the compound which has been described probably having been a mixture of chloride and oxide.