Charge-Shift Bonding—A Class of Electron-Pair Bonds That Emerges from Valence Bond Theory and Is Supported by the Electron Localization Function Approach

This paper deals with a cen- tral paradigm of chemistry, the elec- tron-pair bond. Valence bond (VB) theory and electron-localization func- tion (ELF) calculations of 21 single bonds demonstrate 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 den- sity plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RECS, and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contribu- tions to the bond energy. The paper de- fines the atomic propensity and territo- ry for CS bonding: Atoms (fragments) that are prone to CS bonding are com- pact electronegative and/or lone-pair- rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and in- volves: a) homopolar bonds of heteroa- toms with zero static ionicity, b) heter- opolar s and p bonds of the electro- negative and/or electron-pair-rich ele- ments among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental mani- festations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reac- tions as compared to hydrogen-trans- fers.