Chalcogen nucleophilic attack

E = S, Se X = I, Br) cation.20 Whether this cation really forms in solutions, especially in low-polar solvents, is difficult to prove. It has been shown, however, that the calculated NBO charge distribution on [LE-X]+ can be of great help in predicting the most likely product, at least among [LE-EL]2+ dications, C.-T. spoke , and T-shaped hypervalent adducts.21,22 In fact, these three types of products can formally derive from a nucleophilic attack of the appropriate nucleophile on the [LE-X]+ cation at the chalcogen or the halogen site. 

The molecular constraint which precludes ligand exchange unambiguously ensures that the tricyclic compounds undergo nucleophilic attack exclusively at the pseudo-equatorial carbon atom, irrespective of the nature of the carbon atom situated in the apical ligand of the TBP. The compounds 1-4 were obtained by alkylation of the chalcogen atom of the corresponding bicyclic (thio)phosphate, 6 and 7, with trialkyloxonium tetrafluoroborate at -78 °c (2). 

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. ... 

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