Hypercoordinated species

Nucleophilic Substitution on Silicon Stable Hypercoordinated Species... 

All these experiments illustrate the great reactivity of hypervalent species. They confirm the possibility of pentacoordinated intermediates in the nucleophilic activation. This possibility cannot be ruled out only on the basis of the argument of a more crowded and less electrophilic species than tetracoordinated silicon. Furthermore, after these results, it becomes interesting to understand why these hypercoordinated species react faster than the tetracoordinated species. Two possible explanations are the increase in the length of Si —X bonds which corresponds to a higher lability and the increase of the electrophilicity of the central silicon atom. 

BRIDGING HYPERCOORDINATE SPECIES WITH DONOR ATOM PARTICIPATION... 

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... 

S has only two unpaired electrons. By contrast, oxygen is almost exclusively mono-or divalent OH and H2O are stable but OF4 and OFg are not. Other examples of small stable (or metastable) hypervalent fluoride compounds include PFj, CIF3, CIF5, and XeF,j, (n=2, 4, 6). Radicals such as SFj, SF5, and PF4 have also been observed but are difficult to study in the laboratory because of their high reactivity. Nonetheless, they represent hypervalent or hypercoordinated species. 

The structure of CHj, also known as the methonium ion (Figure 5.35), has been the subject of extensive theoretical investigation. As originally proposed by Olah, the structure is a hypercoordinate species with a three-center, two-electron (3c-2e) bond. ... 

The hypercoordinated (9-M-(n 4- 1)) radicals of the general formula (MH + i) may or may not be stable, with respect to the dissociation to a hydrogen atom and the normal-coordinated MH molecule, depending on the nature of the M atom. Three cases may be observed, as shown in 38-40. In 38 the hypercoordinated species is more stable than the normal-coordinated constituents, while in 39 the species is metastable, and unstable in 40. 

Table 3 Ionization potentials of the central atom of some hypercoordinated species and their dissociation energies to normal-valent species +2 F. Ail energies in kcai moi ...

As an illustration of this point, Table 3 reports the first and second ionization potentials (IPs) for the central atom of some selected neutral hypercoordinated species of the type AF , together with the stability of these species with respect to dissociation of the hypervalent bonds. It can be seen that even if the first IP is an important parameter for the stability of AF , the second IP is at least as much important and indeed marks the limit between stable and unstable systems and in particular the first-row systems (NF5, OF4, F4, NeF2). The reason for the first-row exception is therefore quite clear within the present VB model. 

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