Application of three-center-four-electron bonding for structural and stability predictions of main group hypervalent molecules: the fulfillment of octet shell rule

摘要

We show that the structures and stabilities of main group closed-shell hypervalent molecules with five and six valence electron pairs around the central atom can be satisfactorily rationalized using a three-center-four-electron (3c-4e) bonding scheme, together with the sp~2 - and sp-hybridizations, respectively. These bonding schemes eliminate the use of d-orbitals for the bonding description of the main group hypervalent species as required in the conventional sp~3d - and sp~3 d~2-hybridizations. The 3c-4e bonding schemes presented here are consistent with high-level ab initio calculations which show d-orbital participation in main group hypervalent molecules is insignificant, and are thus more correct than the sp~3d - and sp~3d~2-hybridization schemes. An important conclusion based on the 3c-4e bonding schemes is that the central atom in these molecules, while seemingly in violation of the octet rule according to the electron-dot formula, actually fulfills the octet shell. The fulfillment of the octet electron shell for the central atom is achieved by discounting the non-bonding electrons which are localized at the ligands, and is interpreted as a factor in the stability of these closed-shell hypervalent molecules. To physically withhold the non-bonding electrons from being shared with the central atom, the electronegativity of the ligands must be significantly greater than that of the central atom. This minimizes the repulsive interactions between the non-bonding electron lobes and other bonded electron pairs. This conclusion is in accord with the experimental observation that nearly all stable main group hypervalent molecules have a central atom more electropositive than its ligands.