pi 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 calcul ating pi-bond energies in the double-bonded species H(m)A=B H-n, where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually asso ciated with obtaining pi-bond strengths by targeting only the pi bond, whil e all other factors remain constant. In this manner, a clean separation bet ween sigma- and pi effects can be achieved which highlights some expected t rends in bond strength upon moving from left to right and up and down the P eriodic Table. Intra-row pi bonds conform to the classic statement by Pauli ng [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 pi bonds do not. This variance with Pauling's statement is shown to be due to the const raining effect of the underlying sigma bonds which prevents optimal p(pi)-p (pi) overlap. While Pauling's statement was based on the assumption that th e resonance energy (RE) would be large for heteronuclear and small for homo nuclear bonds, we have found large REs for all bonds studied herein; this l eads to the conclusion that REs are dependent not only on the electronegati vity difference but also the electronegativity sum of the constituent atoms . This situation where the bond is neither covalent nor ionic but originate s in the covalent-ionic mixing has been termed charge shift (CS) bonding [S . Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Sec. 1992, 114, 78 61], We have shown that CS bonding extends beyond single sigma bonds in fir st row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.