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 io nic structures of the bond. This phenomenon is not associated with bond pol arity, and charge-shift bonds exist among homonuclear (e.g., F-2, O-2) as w ell 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.; Hibe rty, 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-M aksic, M., Eds.; Ellis-Horwood, New York, 1991]. Valence bond (VB) computat ions performed on MH3-Cl (M = C, Si, Ge, Sn, Pb) show that M-Cl is a "charg e-shift bond" for which the major contribution to bonding arises from the r esonance energy between the covalent M . - . Cl (1) and ionic M+:Cl- (2) st ructures. The computations show that the strongest bond is Si-Cl while C-Cl is the weakest or the second weakest in the series. ia detailed analysis s hows that the root cause for the emergence of charge-shift bonding and the associated chemical manifestations is the joint behavior of the covalent an d ionic VB constituents. Thus, repulsive interactions raise the covalent st ructure in energy, while enhanced electrostatic stabilization along with so me pi-back-bonding lowers the energy of the ionic structure. The covalent b onding is so meager that the major source of bonding must arise from the co valent-ionic resonance energy, i.e., the charge-shift resonance. Thus, for example, the root cause of the strong Si-Cl bonding originates in the energ y proximity of its constituent VB structures and the near coincidence of th eir energy minima, which lead to a very large charge-shift resonance energy . Due to the large resonance energy, charge-shift bonds may possess high io nic charge distribution, but their ionicity remains virtual with no practic al 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 tendenc y of Sn and Si to form hypercoordination. The generality of this paradigm i s discussed.