Comparison of steric hindrance in silylenium and carbenium cations and their complexes

Steric effect in the complexes of tri-coordinate silylenium and carbenium ions with model nucleophiles is discussed based on calculated energies of complex formation and on natural steric analysis (a part of the NBO theory). While the energies of complexation of CH 3 + are greater than those of SiH 3 + , for trimethyl-substituted cations the order is reversed. This observation is interpreted in terms of smaller steric hindrance of trimethylsilyl cation compared to t -butyl cation. Natural steric analysis points to the potentials and difficulties in separate treatment of steric and electron delocalization effects on stabilization of these species.