Hypervalent sulfur species

However, on account of the particularly short C-S distances, multiple bonds (probably of the d-si-p-si type) between hypervalent sulfur and carbon occur in a number of species. 

In applying an electron-pair bond model for the hypervalent molecules studied, the (apical, equatorial) and (equatorial, equatorial) isomers, namely 23-eq and 23b, respectively, are both local minima. This means that the tetracoordinated species are less strained than the pentacoordinated ones, due to less antibonding properties of the electron delocalization between the geminal ring bonds on the hypervalent sulfur atom. Yet, the more strained (eq, eq) isomers are, surprisingly, more stable than the (ap, eq) isomers in most of the hypervalent three-membered rings studied <2001PCA10711>. 

In organic sulfur chemistry, these decet species have long been considered to be transition states or unstable intermediates in the nucleophilic substitution reaction (SN2), and not to exist as real molecules except in a few cases [13]. However, recently numerous heteroatom compounds containing hypervalent structures have proved to be of considerable interest to many organic chemists. 

Explanations for the existence of hypervalent species started with Pauling s proposal of d orbital hybridization [3] where, for example, a set of sp d hybrid orbitals on sulfur was put forward to account for the hexavalence of SFg. Chemical computations [4] ruled out the participation of d atomic orbitals. A later model, the Rundle-Pimentel three-center, four-electron (3c-4e) bonding model [5], does not require d orbital participation and, so, is consistent with chemical computations. Other hypervalency models include the diabatic state model of Dixon and coworkers [6] and the democracy principle of Cooper and coworkers [7]. These various models provide useful insights into some aspects of the electronic structure of hypervalent molecules, but they don t provide an overarching description of these molecules that enables connections to be drawn between hypervalency and related molecular phenomena. 

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