π bonding in second and third row molecules: Testing the strength of Linus's blanket

The flexibility of valence bond (VB) theory provides a new method of calculating π-bond energies in the double-bonded species H_mA=BH_n, where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining π-bond strengths by targeting only the π bond, while all other factors remain constant. In this manner, a clean separation between σ- and π effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row π bonds conform to the classic statement by Pauling [L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row π bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying σ bonds which prevents optimal pπ-pπ overlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent - ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single σ bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.