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

Abstract

This paper deals with a central paradigm of chemistry, the electron-pair bond. Valence bond (VB) theory and electron-localization function (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 density plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RE(CS), 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 contributions to the bond energy. The paper defines the atomic propensity and territory for CS bonding: 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 involves: a) homopolar bonds of heteroatoms with zero static ionicity, b) heteropolar sigma and pi 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), c) all hypercoordinate molecules. Several experimental manifestations 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 reactions as compared to hydrogen-transfers.

VSEPR Theory: A closer look at chlorine trifluoride, ClF3.

“…3. Basins 11+19 and 10+15 (similar basin splitting is observed for F2 [2]) each total 0.91e; again less than a pair (Wiberg index 0.63). So the three Cl-F bonds are 4.…”

A survey of recent developments in ab initio valence bond theory

“…These features are common to the VBSCF, BOVB and VBCI methods (see below), and their use has resulted in some new qualitative concepts. [106][107][108] Moreover, owing to the use of strictly localized active orbitals, these methods are suitable for calculating clearly defined diabatic states that are required to retain the physical content of a given asymptotic state, at any point of a reaction coordinate, without collapsing to the ground state by virtue of uncontrolled orbital optimization. 25,109,110 The Valence Bond Self Consistent Field Method…”

“…One such paradigm is called ''charge-shift bonding'' and concerns two-electron bonds which are neither covalent nor ionic but with bonding energy that is dominated by the covalent-ionic resonance interaction; for example FÀ ÀF and OÀ ÀO are charge-shift bonds. 107,154 Another paradigm is the ''ferromagnetic-bonding'' that occurs in high-spin clusters, e.g., n+1 Li n , n+1 Cu n , etc. that are devoid of an electron pairs but have significant bond energy that can reach as much as 19 kcal/mol per atom.…”

The V state of ethylene: valence bond theory takes up the challenge

“…However, it has always been our experience that some ab initio VB methods, providing static and dynamic correlations [9][10][11][36][37][38], are able to give quantitative results as well as physical insight by means of very compact wave functions, with a very small number of chemically lucid VB structures. Examples involve dissociation reactions of twoelectron bonds [9,39] and three-electron bonds [40], reaction barriers [41,42], transition energies [43], and so on [7]. Thus, in view of the difficulties encountered by many researchers and the elaborate strategies that had to be devised to describe the V state of ethylene with MO-CI methods, we decided to address the problem by the use of ab initio VB methods, with the aim of getting accurate values of N ?…”