Charge-shift bonding in group IVB halides: A valence bond study of MH₃- Cl (M = C, Si, Ge, Sn, Pb) molecules

Charge-shift bonds form a distinct class of bonds where all or most of the bond energy is provided by the resonance energy between the covalent and ionic structures of the bond. This phenomenon is not associated with bond polarity, and charge-shift bonds exist among homonuclear (e.g., F₂, O₂) as well as heteronuclear cases [Sini, G.; Maitre, P.; Hiberty, P.C.; Shaik, S.S. J. Mol. Struct. (THEOCHEM) 1991, 229, 163. Shaik, S.; Maitre, P.; Sini, G.; Hiberty, P.C.J. Am. Chem. Soc. 1992, 114, 7861. Lauvergnat, D.; Hiberty, P.C.; Danovich, D.; Shaik, S.J. Phys. Chem. 1996, 100, 5715. Shaik, S.S. In Molecules in Natural Science and Medicine; Maksic, Z.B., Eckert-Maksic, M., Eds.; Ellis-Horwood, New York, 1991]. Valence bond (VB) computations performed on MH₃-Cl (M = C, Si, Ge, Sn, Pb) show that M-Cl is a 'charge- shift bond' for which the major contribution to bonding arises from the resonance energy between the covalent M·-·Cl(1) and ionic M⁺:Cl^- (2) structures. The computations show that the strongest bond is Si-Cl while C- Cl is the weakest or the second weakest in the series. A detailed analysis shows that the root cause for the emergence of charge-shift bonding and the associated chemical manifestations is the joint behavior of the covalent and ionic VB constituents. Thus, repulsive interactions raise the covalent structure in energy, while enhanced electrostatic stabilization along with some π-back-bonding lowers the energy of the ionic structure. The covalent bonding is so meager that the major source of bonding must arise from the covalent-ionic resonance energy, i.e., the charge-shift resonance. Thus, for example, the root cause of the strong Si-Cl bonding originates in the energy proximity of its constituent VB structures and the near coincidence of their energy minima, which lead to a very large charge-shift resonance energy. Due to the large resonance energy, charge-shift bonds may possess high ionic charge distribution, but their ionicity remains virtual with no practical expression. Manifestations of charge-shift bonding are discussed, i.e., the rarity of free R₃M⁺ cations for M = Si, Ge, Sn, and Pb, and the tendency of Sn and Si to form hypercoordination. The generality of this paradigm is discussed.