Low-temperature ionization of neutral hexacoordinate to ionic pentacoordinate silicon complexes. Unusual temperature, solvent and substituent effects

Neutral hexacoordinate complexes of silicon (1) ionize to pentacoordinate siliconium chloride salts (2) in CH2Cl2, CHCl3 or CHFCl2 solutions. 29Si NMR measurements prove that the extent of ionization increases as the temperature of the solution is lowered. The ionizations are slightly exothermic, and have negative entropies. It is shown that increased solvent polarity drives the reaction to the ionic side, i.e. that ion solvation is responsible for the ionization. The lack of a common‐ion effect on addition of tetrabutylammonium chloride indicates the formation of unseparated ion pairs. Copyright © 2002 John Wiley & Sons, Ltd.