Tellurium - Compounds

Compounds

See also category: Tellurium compounds

Tellurium belongs to the same chemical family as oxygen, sulfur, selenium and polonium: the chalcogen family. Tellurium and selenium compounds are similar. It exhibits the oxidation states −2, +2, +4 and +6, with the +4 state being most common.

Tellurides

Reduction of Te metal produces the tellurides and polytellurides, Te_n2–. The −2 oxidation state is exhibited in binary compounds with many metals, such as zinc telluride, ZnTe, formed by heating tellurium with zinc. Decomposition of ZnTe with hydrochloric acid yields hydrogen telluride (H₂Te), a highly unstable analogue of the other chalcogen hydrides, H₂O, H₂S and H₂Se:

ZnTe + 2 HCl → ZnCl₂ + H₂Te

H₂Te is unstable, whereas salts of its conjugate base – are stable.

Halides

The +2 oxidation state is exhibited by the dihalides, TeCl₂, TeBr₂ and TeI₂. The dihalides have not been obtained in pure form, although they are known decomposition products of the tetrahalides in organic solvents, and their derived tetrahalotellurates are well-characterized:

Te + X₂ + 2 X− → TeX2−
4

where X is Cl, Br, or I. These anions are square planar in geometry. Polynuclear anionic species also exist, such as the dark brown Te
2I2−
6, and the black Te
4I2−
14.

Fluorine forms two halides with tellurium: the mixed-valence Te₂F₄ and TeF₆. In the +6 oxidation state, the –OTeF₅ structural group occurs in a number of compounds such as HOTeF₅, B(OTeF₅)₃, Xe(OTeF₅)₂, Te(OTeF₅)₄ and Te(OTeF₅)₆. The square antiprismatic anion TeF2−
8 is also attested. The other halogens do not form halides with tellurium in the +6 oxidation state, but only tetrahalides (TeCl₄, TeBr₄ and TeI₄) in the +4 state, and other lower halides (Te₃Cl₂, Te₂Cl₂, Te₂Br₂, Te₂I and two forms of TeI). In the +4 oxidation state, halotellurate anions are known, such as TeCl2−
6 and Te₂Cl2−
10. Halotellurium cations are also attested, including TeI+
3, found in TeI₃AsF₆.

Oxocompounds

Tellurium monoxide was first reported in 1883 as a black amorphous solid formed by the heat decomposition of TeSO₃ in vacuum, disproportionating into tellurium dioxide, TeO₂ and elemental tellurium upon heating. Since then, however, some doubt has been cast on its existence in the solid phase, although it is known as a vapor phase fragment; the black solid may be merely an equimolar mixture of elemental tellurium and tellurium dioxide.

Tellurium dioxide is formed by heating tellurium in air, causing it to burn with a blue flame. Tellurium trioxide, β-TeO₃, is obtained by thermal decomposition of Te(OH)₆. The other two forms of trioxide reported in the literature, the α- and γ- forms, were found not to be true oxides of tellurium in the +6 oxidation state, but a mixture of Te4+, OH− and O−
2. Tellurium also exhibits mixed-valence oxides, Te₂O₅ and Te₄O₉.

The tellurium oxides and hydrated oxides form a series of acids, including tellurous acid (H₂TeO₃), orthotelluric acid (Te(OH)₆) and metatelluric acid ((H₂TeO₄)_n). The two forms of telluric acid form tellurate salts containing the TeO2–
4 and TeO6−
6 anions, respectively. Tellurous acid forms tellurite salts containing the anion TeO2−
3. Other tellurium cations include TeF2+
8, which consists of two fused tellurium rings and the polymeric TeF2+
7.

Zintl cations

When tellurium is treated with concentrated sulfuric acid, it forms red solutions containing the Zintl ion, Te2+
4. The oxidation of tellurium by AsF₅ in liquid SO₂ also produces this square planar cation, as well as with the trigonal prismatic, yellow-orange Te4+
6:

4 Te + 3 AsF₅ → Te2+
4(AsF−
6)₂ + AsF₃

6 Te + 6 AsF₅ → Te4+
6(AsF−
6)₄ + 2 AsF₃

Other tellurium Zintl cations include the polymeric Te2+
7 and the blue-black Te2+
8, which consists of two fused 5-membered tellurium rings. The latter cation is formed by the reaction of tellurium with tungsten hexachloride:

8 Te + 2 WCl₆ → Te2+
8(WCl−
6)₂

Interchalcogen cations also exist, such as Te₂Se2+
6 (distorted cubic geometry) and Te₂Se2+
8. These are formed by oxidizing mixtures of tellurium and selenium with AsF₅ or SbF₅.

Organotellurium compounds

Tellurium does not readily form analogues of alcohols and thiols, with the functional group –TeH and are called tellurols. The –TeH functional group is also attributed to using the prefix tellanyl-. Like H₂Te, these species are unstable with respect to loss of hydrogen. Telluraethers (R-Te-R) are more stable as are telluroxides.