New landscape of electron-pair bonding: Covalent, ionic, and charge-shift bonds

We discuss here the modern valence bond (VB) description of the electron-pair bond vis-à-vis the Lewis-Pauling model and show that along the two classical families of covalent and ionic bonds, there exists a family of charge-shift bonds (CSBs) in which the "resonance fluctuation" of the electronpair density plays a dominant role. A bridge is created between the VB description of bonding and three other approaches to the problem: the electron localization function (ELF), atoms-in-molecules (AIM), and molecular orbital (MO)-based theories. In VB theory, CSB manifests by repulsive or weakly bonded covalent state and large covalent-ionic resonance energy, RE_CS. In ELF, it shows up by a depleted basin population with fluctuations and in AIM by a positive Laplacian. CSB is derivable also from MO-based theory. As such, CSB 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 CSB and outlines its territory: Atoms (fragments) that are prone to CSB are compact electronegative and/or lone-pair-rich species. As such, the territory of CSB transcends considerations of static charge distribution, and it involves (a) homopolar bonds of heteroatoms with zero static ionicity, (b) heteropolar σ- and π-bonds of the electronegative and/or electron-pairrich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc.), and (c) electron-rich hypercoordinate molecules. Several experimental manifestations of charge-shift bonding are discussed.