Chalcogens 3-center 4-electron bonding

Once the thiol is introduced to the coordination sphere of the selenoxide or telluroxide, a second slower reaction occurs. This step is associated with reduction of the chalcogen(IV) oxidation state to the chalcogen(II) oxidation state, which was demonstrated with dihydroxy telluranes 52 and 53. In the tellurium(IV) oxidation state of 52 and 53, the 5p orbital of tellurium is involved in the three-center, four-electron bond and cannot interact with the carbon 7r-framework. Long-wavelength absorption maxima for 52 and 53 are found at 510 and 500 nm, respectively in water. Reductive elimination generates a tellurium(II) atom, whose 5p orbital can now... 

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). ... 

Fig. 39). The ESR data for dimethylselenide dimer cation radical 107c are consistent with two-center three-electron bonding, in which two electrons are in a cr bond between the two chalcogens, while the third electron is in a o orbital. 

Bis- and tris-chalcogen monoxides when treated with Tf20 afford di- or tri-chalcogona dications, respectively these salts can be considered 2-center, 2-electron or 3-center, 4-electron bonded species (eq 85). 432 xjjgge compounds are susceptible to nucleophilic attack at carbon or at the chalcogen and can act as alkylating agents under very mild conditions. ... 

In contrast to cyclic tetrachalcogenadiynes [75], the conformations of cydic tetra-chalcogenatetraynes [76] are not mainly determined by electronic effects. In the smaller counterparts (cydic tetrathia- and tetraselenadiynes) where only one triple bond links the chalcogen centers the repulsive interaction between the lone pairs of the chalcogens is the structure-determining interaction [75, 76]. This repulsive interaction is remarkably reduced by the longer distance spanned by the two acetylene units. The favored perpendicular conformation is circumvented by steric or... 

As discussed earlier, aU of the reactirais in eqs. (2-6) have been proposed to proceed via Pd catalytic cycles. Thus, it is of great interest and potential relevance to establish the viability of carbon-chalcogen bond-forming reductive eliminatirai from Pd centers. In addition, it is important to understand the effects of steric and electronic factors as well as the influence of ancillary ligands on the mechanisms of reductive elimination from Pd to design improved catalytic transformations. 

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