Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects

Abstract

This essay describes the successive births of valence bond (VB) theory during 1916–1931. The alternative molecular orbital (MO) theory was born in the late 1920s. The presence of two seemingly different descriptions of molecules by the two theories led to struggles between the main proponents, Linus Pauling and Robert Mulliken, and their supporters. Until the 1950s, VB theory was dominant, and then it was eclipsed by MO theory. The struggles will be discussed, as well as the new dawn of VB theory, and its future.

On the Nature of the Bonding in Coinage Metal Halides

“…This allowed essentially the same accuracy as using the full basis set but with a significantly lower computational cost. The VB calculations were performed at two levels: with the VB Self-Consistent-Field (VBSCF) [11][12][13][14] and the Breathing Orbital VB (BOVB) methods [41][42][43]. These methods belong to the classical VB approach, in which the active orbitals are constrained to remain "strictly localized" on a given atom-i.e., they are only allowed to expand on the functions of the basis set that are centered onto one specific atom.…”

“…All these particularities lead to substantial correlation effects on transition metal bonding, often involving a combination of different types of correlation which in return require an adequate and elaborate theoretical treatment. With a few notable exceptions [2,6,7,10], ab initio classical Valence Bond (VB) theory [11][12][13][14] has not dealt with this important area of chemistry. From their intimate connection with Lewis structures and the electron pair bond concept, classical VB wave functions can provide intuitive and detailed insight into the nature of chemical bonding.…”

Classical and Quantum Orbital Correlations in the Molecular Electronic States

“…Namely, the electrons occupy delocalized orbitals made from the quantum superposition of atomic orbitals. These two quantum mechanical theories are equivalent to each other at the limit [4,5], and both explain the stability of molecules as a decrease in energy due to the principle of quantum superposition.…”

A Critical Look at Linus Pauling’s Influence on the Understanding of Chemical Bonding

“…[ 13 ]. The erroneous equalization of a classical model with a quantum theoretical treatment of chemical bonding was repeated in other places [ 103 , 104 ] and it is the source of misleading statements. VB theory is a quantum chemical approach for the mathematical description of the electronic structure of molecules, which was introduced by Heitler and London in 1927 [ 10 ].…”

“…Chemists trained and accustomed to the model of resonance introduced by Pauling may still find it useful to use VB calculations, but they forgo all the information given by the symmetry of the wave function [ 124 , 125 ]. However, it is fair to say that problems like the homolytic bond breaking of electron-pair bonds are more easily dealt with using VB calculations, which remains an important approach in the toolbox of quantum chemical methods [ 99 , 100 , 101 , 102 , 103 , 104 , 126 , 127 ]. The VB and MO methods have been compared and discussed in several papers, which shed further light on the two approaches [ 128 , 129 , 130 , 131 , 132 , 133 , 134 ].…”