Compounds of Tellurium

In its chemical behaviour tellurium shows considerable resemblance to sulphur and selenium, and its inclusion in the same group with these elements was due in the first place to the analogy in the composition of many of its compounds with that of compounds of sulphur and selenium. There are, however, important characteristics which differentiate the compounds of tellurium from those of the other elements of the group. Like sulphur and selenium, the element exhibits bi-, quadri- and sexa- valency, but those compounds containing tellurium in the quadrivalent condition are the most stable. Moreover, in this condition tellurium appears to resemble platinum somewhat, there being evidence that the four valencies are directed symmetrically about the atom in a simple square configuration, possibly in one plane; thus compounds such as dimethyltelluronium dihalides, TeMe₂X₂ (X = Cl, Br or I), are known to exist in two distinct isomeric forms, α- (trans-) and β- (cis-) forms, although it must be mentioned that the molecular structure of these compounds is a matter of controversy, and conductivity measurements show that in aqueous solution one of the halogen atoms is hydrolysed more or less completely, whilst the other atom forms a halogen ion, for example:

[Te⁺Me₂Cl]Cl^- + H₂O ⇔ [Te⁺Me₂OH]Cl^- + H⁺Cl^-.

The basic tendencies of tellurium also lead to important characteristic differences in the properties of its compounds compared with the properties of its sulphur and selenium analogues.

Tellurium forms only one compound with hydrogen, hydrogen telluride, H2Te, which is a gas at ordinary temperatures and resembles hydrogen sulphide in its ability to precipitate as tellurides many heavy metals from their solutions, although the conditions of such precipitation do not appear to have been fully investigated. As already described, many of the tellurides occur in nature. With each of the halogens tellurium forms a tetrahalide of the type TeX₄, which with water produces tellurous acid. The tetrahalides show a marked tendency to form additive compounds, and except in the case of the fluoride, yield with alkali halides co-ordinative compounds of the type R₂TeX₆, analogous with the platinum compounds. The only other halides known are the hexafluoride, TeF₆, the dichloride, TeCl₂, and the dibromide, TeBr₂, the last two being very unstable under ordinary conditions of temperature and pressure. Certain indefinite oxyhalides have also been isolated.

Oxides of composition TeO, TeO₂ and TeO₃ are known, as well as a fourth compound, Te₃O₇, which appears to be a basic tellurium tellurate, 2TeO₂.TeO₃. The dioxide is only slightly soluble in water and the aqueous solution does not redden litmus; it dissolves in aqueous alkalis with formation of tellurites, from solutions of which tellurous acid, H₂TeO₃, may be precipitated. This compound is also only slightly soluble in water, but its solution is sufficiently acid to redden litmus. The acid is very unstable and readily loses water to form the dioxide; this tendency to dehydration is characteristic of acids derived from elements with weak metallic properties, the oxides of which are feeble anhydrides. That the dioxide also has definite basic properties is shown by its reaction with many acids to produce salt-like compounds containing quadrivalent tellurium, as, for example, the tetrahalides and the sulphates. Tellurium trioxide, obtained by heating telluric acid, is unacted on by water, and is readily reduced to the dioxide. Telluric acid, which is produced by the action of powerful oxidising agents on tellurium, is a very weak acid, but it is a more effective oxidising agent than sulphuric acid and, like selenic acid, is able to liberate chlorine from hydrochloric acid. Telluric acid shows a greater tendency than its sulphur and selenium analogues to enter into the formation of complex compounds such as heteropoly-acids.

The few compounds of tellurium with the elements sulphur, carbon and nitrogen which have been investigated. Most tellurium compounds are colourless, but the sulphur compounds, as well as the polytellurides, are in general dark red. A similar deepening of colour occurs in the polysulphides and polyselenides.

Tellurium and iodine, in the molten condition, are iniscible in all proportions, and the system tellurium-iodine has been examined from the thermo-analytical standpoint, the freezing-point curve giving indications only of the formation of a tetra-iodide, TeI₄, in the fused mixture.

A tellurium di-iodide has been described by Berzelius as obtained when tellurium and iodine are sublimed together, but its existence as a definite compound appears doubtful. Damiens has shown that the so-called tellurium di-iodide is a mixture of the tetra-iodide and a solid solution of tellurium with the tetra-iodide.

Tellurium and selenium are miscible in all proportions when molten, no combination apparently occurring.

A solution of tellurium-selenium sesquioxide, TeSeO₃, has been described. A basic tellurium selenate, 2TeO₂.SeO₃, analogous to the foregoing sulphur compound, is obtainable from tellurium oxide and selenic acid.

A tellurium analogue of carbon disulphide has not been isolated. A product described as carbon ditelluride has been shown to consist of the sulphidotelluride and its decomposition products.