Tellurium 3-center 4-electron bonding

In the tellurium(IV) oxidation state, the 5p orbital of the tellurium(fV) atom is involved in the three-center, four-electron bonding to the halides and cannot interact with the carbon rr-framework. Long-wavelength absorption maxima for 6-11 are found between 515 and 565 nm in dichlromethane. Reductive elimination generates the tellurium(II) oxidation state in compounds 12 and 13. The tellurium(II) 5p orbital can now overlap with the carbon 7r-framework and long-wavelength absorption maxima for 12 and 13 are observed between 765 and 830 nm in dichloromethane. The rate of loss of the 515-565 nm band and the rate of appearance of the 765-830 nm band are identical, which is consistent with reductive elimination as the rate-determining step of this process. 

The corresponding selenium(IV) derivatives are less stabilized by the three-center, four-electron bonds in the chalcogen(rV) complex than the tellurium(lV) derivatives. As a consequence, the selenium(lV) derivative is more easily reduced than the tellurium(lV) derivative. Diphenylselenium(lV) dibromide (1) undergoes a two-electron reduction with Ep at - - 0.40 V (vs. SCE) while diphenyltellurium(lV) dibromide (2) undergoes a two-electron reduction with at-1-0.05 V (vs. SCE). ... 

Each 15-electron V(CO)4(dppe) fragment requires three electrons to satisfy the effective atomic number rule, which clearly requires a degree of multiple bonding between the tellurium and vanadium centers. Two alternative descriptions, A and B, were proposed as representations for the bonding. 

A large number of tellurium(II)-complexes exist also with an electron-rich three-center bond 12 7). In most complexes the tellurium(II) shows squeire planar four coordination. 

This family of compounds has been extensively studied in recent years. Wheeler has analyzed the electronic structure of Ni9(//4-Te)6(PEt3)8 with EH calculations. Hoffmann and coworkers have also used EH calculations to examine the electronic structure of this molecular cubic cluster to draw relationships with nickel-tellurium extended structures. More recently, we have performed EH and Xa calculations to rationalize the bonding in these clusters, as a function of different parameters such as the electron count or the nature and the size of the different elements constituting the cluster cage. " For symmetry reasons, a localized two-center-two-electron... 

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