Silylenium ions structure

Quantum mechanical calculations of silylenium ion structures have been made mostly by ab initio methods. Although the results seem to depend to some extent on the procedure used, they provide important information concerning the structure and reactivity of silylenium ions. 

Calculations revealed that the parent structure of the silylenium ion H3Si+ is more stable than that of H3C+ by 41-73.5 kcal/mol. However AE tends to decrease strongly on replacement of the hydrogen atom, indicating that substituents, in particular it donors, are decidedly more effective in stabilizing carbenium ions than in stabilizing silylenium ions. Thus, the methyl cation is stabilized by an amino substituent by 93.8 kcal/mol but the silyl cation by only 38.3 kcal/mol (27). +... 

Optimization of geometry for some silylenium cations was made (22,24). Such calculations predict that silylenium ions will adopt a planar structure in contrast to silyl anions, which are predicted to be pyramidal. It should be noted that theoretical studies also indicate a high ability of silicon to accommodate negative charge. The parent silyl anion H3Si was calculated to be more stable than its carbon analog by about 50 kcal/mol (24). This implies a remarkable affinity of silylenium ions toward electron-rich species. 

One of the key observations in understanding the structure of these ions in solution was 35C1NMR spectroscopy. An ionic perchlorate should exhibit a sharp 35C1 resonance, because in this tetrahedral environment quadrupolar relaxation is ineffective. Association of the silylenium ion with perchlorate, either as an ion pair or as a covalent species, would result in considerable line broadening. Chlorine-35 linewidths can be as... 

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