Comparison of C−Cl and Si−Cl Bonds. A Valence Bond Study

VB calculations with breathing orbitals (BOVB) show that the H3Si−Cl and H3C−Cl bonds are qualitatively different. The differences are rooted in the properties of the H3Si+ and H3C+ species. Thus, the H3C+ cation has an evenly distributed charge and relatively large ionic radius, and therefore the cation maintains a long distance from the counterion Cl-. Consequently, the ionic−covalent mixing remains of secondary influence and shortens slightly the RCCl distance in agreement with the Pauling recipe for polar bonds. On the other hand, in H3Si+ the charge is highly localized on silicon. Consequently, the cation acquires a diminished effective size along the missing coordination site. This allows a close approach of Cl- as well as a very large electrostatic interaction between the Si+ and Cl- centers in the ionic VB structure. Consequently, the ionic potential energy curve R3Si+Cl- approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si−Cl bond a true charge-sh...