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 of the convalent Heitler-London and the ionic contributions to the bond energy. While a number of bonds (H-H, H 3 C-H, H 3 Si-H, Li-Li, Na-F) are found to correspond to the traditional covalent (Heitler-London) or ionic pictures, some other bonds, even homopolar ones (H 2 N-NH 2 , HO-OH, F-F), have an unbound or weakly bound covalent component