Donor-Stabilized Silyl Cations. 3. Ionic Dissociation of Hexacoordinate Silicon Complexes to Pentacoordinate Siliconium Salts Driven by Ion Solvation¹

Neutral hexacoordinate silicon complexes 1 undergo an equilibrium dissociation to ionic siliconium chlorides 6 at low temperatures in polar solvents:  chloroform, dichloromethane, and fluorodichloromethane. The extent of dissociation increases with decreasing temperature, despite the formation of two ions from each molecule. The reaction enthalpies and entropies for the ionizations are negative, and their absolute values increase with increasing solvent polarity, indicating that solvation of the ions drives the dissociation process. The position of the equilibrium is readily controlled by variation of solvent polarity, temperature, replacement of the chloro ligand by better leaving groups (triflate, bromide, or tetrachloroaluminate), and variation of substituents (R) or ligands (X). The results are supported by crystal structures of the siliconium salts:  8a,c,d and 14d.