Silicon cation, tetracoordinate structure

The molecular structure of 1 shown in Fig. 4 demonstrates that the vinyl cation is linear around the dicoordinated C atom, which indicates sp hybridi2ation for the C atom. The C -C bond is unusually short (122.1 pm) and approaches the length of a regular C C triple bond. A quite remarkable feature of the molecular structure of 1 is the unusual length of the C -Si bonds (198.4 and 194.6 pm), around 10 pm longer than regular single bonds between sp -hybridized carbon and tetracoordinated silicon atoms. 

In cases in which the high Lewis acidity of the cationic silicon is decreased by an intramolecular Lewis basic group (LB), as in the tetracoordinated cation 9 (and likewise in the siliconium ion 10 Scheme 2), interactions between the positively charged silicon atom and the solvent and/or anion are of minor importance. The structures, spectroscopic properties, and reactivities of these sUyl cations are greatly determined by the electron-donating ability of the LB groups. As a consequence, the structural and spectroscopic features of silyl cations 21 and 22, which are stabilized by intramolecular electron donation from an aryl substituent, closely resemble those of benzenium ions 11 (Scheme 5) [33, 34]. The intramolecular stabilization operative in silylium ions 9 offers the intriguing possibility of... 

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