Tellurium compounds hexavalent

Tellurium compounds that do not have a tellurium-carbon bond but contain at least one carbon atom in the molecule, in which the tellurium atom is divalent, tetravalent, or hexavalent, have been prepared. The tellurium atom in these compounds is linked to carbon atoms via one or more heteroatoms such as B, Al, Si, Ge, Sn, Pb, P, As, S, Se, and Hg. The derivatives of divalent tellurium can be considered to be derived from the unknown telluroxylic acid (H2Te02), those of tetravalent tellurium from the unknown orthotellurous acid (H4Te04), and those of hexavalent tellurium from the well known orthotelluric acid (H6Te06). 

Orthotelluric acid is a crystalline, well-characterized compound. All the hexavalent tellurium compounds discussed in this chapter are derivatives of orthotelluric acid. Telluric acid (H2Tc04), corresponding to sulfuric acid, is not known. Organic derivatives of such a telluric acid similar, for instance, to dimethyl sulfate, are also unknown. 

Organic tellurium compounds with one Te—C bond containing divalent, tetravalent, or hexavalent tellurium have been prepared. The most widely investigated and best characterized compounds are the diorgano ditellurium derivatives and the organo tellurium trihalides. Pentafluoroethyl tellurium chloride tetrafluoride is the only known example of a hexavalent tellurium compound with one Te-C bond in the molecule. 

The class of organic tellurium compounds with two Te —C bonds contains divalent, tetravalent, and hexavalent tellurium derivatives. 

In many reactions, selenium is an oxidant as well as a reductant. Strong oxidants convert selenium dioxide and its derivatives to the hexavalent state. Although hexavalent selenium compounds are oxidants, these are less active and difficult to reduce. Selenium salts resemble the corresponding sulfur and tellurium salts in behavior. 

Compounds of hexavalent selenium are much more easily reduced than analogous compounds of either hexavalent sulfur or hexavalent tellurium this trend is analogous to that in the scries C (V)—Br(V)—I(V) (p. 224). 

Compounds containing hexavalent tellurium in which the tellurium atom is bonded via one or more heteroatoms to at least one carbon atom can be considered to be condensation products formed from orthotelluric acid and compounds with at least one active hydrogen atom in the molecule. 

II. Compounds with one tellurium-carbon bond. The tellurium atom in such compounds can be di-, tetra-, or hexavalent. 

---