The Valence Bond Perspective of the Chemical Bond

This chapter discusses the valence bond (VB) perspective of the electron-pair bond. It demonstrates that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the 'fluctuation' of the electron-pair density plays a dominant role. A bridge is created between the VB description of bonding and the descriptions by two other approaches, the electron localization function (ELF) and atoms in molecules (AIM) theories. In the VB theory, CS bonding manifests by repulsive or weakly bonded covalent state and large covalent-ionic resonance energy, RECS. In ELF, it manifests by a depleted basin population with fluctuations, and in AIM by a positive Laplacian. CS bonding is shown to be a fundamental mechanism that satisfies the equilibrium condition of bonding, namely, the virial ratio of the kinetic and potential energy contributions to the bond energy. The chapter defines the atomic propensity for CS bonding and outlines its territory: atoms (fragments) that are prone to CS bonding are compact electronegative and/or lone-pair-rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and it involves (i) homopolar bonds of heteroatoms with zero static iconicity, (ii) heteropolar σ- and π-bonds of the electronegative and/or electron-pair-rich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc.), and (iii) hypercoordinate molecules. Several experimental manifestations of CS bonding are discussed, including depleted bonding density, the rare ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reactions as compared to hydrogen transfers.