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 R(CCl) 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 R(3)Si(+)Cl(-) approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si-Cl bond a tote charge-shift bond whose major character is the charge fluctuation inherent in the resonating wave function. The effect of ionicity on the Si-Cl bond length does not follow the Pauling recipe. Indeed, by mixing of the ionic structure the R(SiCl) minimum shifts to a longer distance in comparison with the covalent minimum. The new minimum is simply an intermediate distance between the covalent and ionic minima in keeping with the charge-shift nature of the bond. The manifestations of the diminished effective size of R(3)Si(+) are its strong coordinating ability with electronegative and electron-rich ligands. Implications on the R(3)Si(+) problem are discussed.