Donor-stabilized silyl cations. 12.¹ electrochemical evidence for ionic dissociation of hexacoordinate silicon complexes

Molar conductivities (A_m) were measured for four formally hexacoordinate silicon dichelates (1-4) in CH₂Cl₂, to support previous NMR evidence suggesting ionic dissociation. Concentration dependence of the conductivities at constant temperature revealed substantial ionic conductivity and ion-pair formation in 2 and 3, lower (kinetically controlled) conductivity of 1, and practically no ionic dissociation of 4. Temperature dependence of the molar conductivities shows an increase of A _m of 2 and 3 with increasing temperature, resulting from the decrease in solvent viscosity, as expected from a fully ionic solute. In contrast, the conductivities of 1 and 4 decrease as the temperature is increased, indicating chemical control of ion concentration: decrease in ionic dissociation and predominance of the hexacoordinate silicon form as the temperature is increased. These results are in full agreement with previous ²⁹Si NMR measurements: complexes with either bulky (3) or very good leaving groups (2) as monodentate ligands are essentially fully ionic at room temperature; those with poorer leaving groups or highly electron withdrawing ligands (1, 4) tend to resist dissociation, and the extent of dissociation increases as the temperature is lowered.