THE CHARGE-SHIFT BONDING CONCEPT - ELECTRON-PAIR BONDS WITH VERY LARGE IONIC COVALENT RESONANCE ENERGIES

The bonding mechanism in a variety of electron-pair bonds is studied by means of an ab initio valence bond method specifically designed for a rigorous separation of the covalent Heitler-London and the ionic contributions to the bond energy. While a number of bonds (H-H, H3C-H, H3Si-H, Li-Li, Na-F) are found to correspond to the traditional covalent (Heitler-London) or ionic pictures, some other bonds, even homopolar ones (H2N-NH2, HO-OH, F-F), have an unbound or weakly bound covalent component. These latter bonds do not owe their stability to the low energy of either the covalent or the ionic components, but rather to a very large resonance energy (53-79 kcal/mol) between these valence bond structures and as such are named "charge-shift bonds". Two general observations about electron-pair bonds are shown to be indirect marks of charge-shift bonding. These are Sanderson's "lone-pair bond weakening effect" and the finding of negative standard electron deformation densities in the bonding region. The stabilization brought by charge-shift bonding is shown to derive from the decrease of the electronic kinetic energy at the bonding region. Its magnitude correlates with the compactness of the valence orbitals involved in the bond and is reinforced by the presence of lone pairs adjacent to these orbitals. Larger and larger resonance energies are predicted as the bonded atoms change from left to right and from bottom to top of the Periodic Table. Other trends and features of charge-shift bonds are discussed.