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

VB calculations with breathing orbitals (BOVB) show that the H 3 Si−Cl and H 3 C−Cl bonds are qualitatively different. The differences are rooted in the properties of the H 3 Si + and H 3 C + species. Thus, the H 3 C + 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 H 3 Si + 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 true 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 chargeshift 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.