Nucleophilic Substitution on Silicon Stable Hypercoordinated Species

3 Nucleophilic Substitution on Silicon Stable Hypercoordinated Species 

Based on these differences, it is possible to represent the VBCMDs for typical nucleophilic substitution reactions on Si versus C as shown in Figs. 6.16a and b. In Fig. 6.16a, the ionic curve X SiL3 + X is very stable in the pentacoordinated geometry due to the electrostatic energy of the triple ion structure, much like the case of the (FIIF) species discussed before. Consequently, the pentacoordinated (XSiL3X) species will generally be a 

FIGURE 6.16 The VBCMDs comparing SN2(Si) in (a) versus SN2(C) in (b). The curves in bold are adiabatic state curves (Adapted with permission from Ref. 5.) 

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Nucleophilic substitution on silicon—stable hypercoordinated species Another demonstration of the role of ionic structures is the nucleophilic substitution on Si, which proceeds via pentacoordinated intermediates [81,82], in contrast to the situation in carbon where the pentacoordinated species is a transition state. Recently, Lauvergnat et al. [83], Shurki et al. [84], Sini et al. [85], and Shaik et al. [86] have performed BOVB/6-31G (and a few other basis sets) calculations for a C-X and Si-X bonds (X = H, F, Cl) and made an interesting observation that the minimum of the ionic curve... 

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