molecules Review Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects Sason Shaik 1,* , David Danovich 1 and Philippe C. Hiberty 2,* 1 Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel; [email protected] 2 CNRS, Institut de Chimie Physique UMR8000, Université Paris-Saclay, 91405 Orsay, France * Correspondence: [email protected] (S.S.); [email protected] (P.C.H.) 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. Keywords: valence bond; molecular orbital; Lewis; electron-pair bonds; Pauling; Mulliken; Hund; Hückel 1. Introduction Citation: Shaik, S.; Danovich, D.; This essay tells briefly a story of the emerging two major quantum mechanical theories, Hiberty, P.C. Valence Bond valence bond (VB) theory and molecular orbital (MO) theory, which look as two different Theory—Its Birth, Struggles with descriptions of the same reality, but are actually not. We discuss the struggles between Molecular Orbital Theory, Its Present the two main groups of followers of Pauling and Mulliken, and the ups and downs in the State and Future Prospects. Molecules popularity of the two methods among chemists, and then the fall of VB theory only to be 2021, 26, 1624. https://doi.org/ revived and to flourish. We end the story with the description of the renaissance in modern 10.3390/molecules26061624 VB theory, its current state and its future outlook. The grassroots of Valence Bond (VB) theory date back to the second decade of the 20th Academic Editor: Steve Scheiner century, when Lewis published his seminal paper entitled, “The Atom and The Molecule”[1]. Lewis made use of the discovery of the electron as a fundamental particle of matter, while Received: 19 February 2021 interjecting his command of chemical facts. These led him to conclude that the most Accepted: 10 March 2021 abundant compounds are those which possess an even number of electrons. He therefore Published: 15 March 2021 formulated the “quantum unit of chemical bonding”, an electron pair that glues atoms of most known molecular matter. In so doing, he was brilliantly able to derive electronic structure Publisher’s Note: MDPI stays neutral cartoons that are used to this day and age for teaching and as means of communication with regard to jurisdictional claims in among chemists. Lewis further distinguished between shared (covalent), ionic bonds, and published maps and institutional affil- iations. polar bonds. He also laid foundations for resonance theory, and used it to explain for the first time color in molecules. He even discussed geometry in terms akin to the valence-shell electron pair repulsion (VSEPR) approach [2]. The contribution of Lewis, and its implementation in 1927–1928 into quantum mechan- ics by Heitler and London [3–5], reached Pauling, who was then in Europe in a mission Copyright: © 2021 by the authors. to learn the new quantum mechanics (and bring it back to the USA). Pauling was excited. Licensee MDPI, Basel, Switzerland. He dropped all the previous mechanical models, which he used to teach [6] and began a This article is an open access article wide-ranging program of this theory which he called valence bond theory, and which he distributed under the terms and conditions of the Creative Commons summarized in his monograph [7]. Pauling’s work translated Lewis’ ideas to quantum Attribution (CC BY) license (https:// mechanics, and the work received very high attention and became extremely popular creativecommons.org/licenses/by/ among chemists. 4.0/). Molecules 2021, 26, 1624. https://doi.org/10.3390/molecules26061624 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 24 Molecules 2021, 26, 1624 2 of 24 Molecular orbital (MO) theory was developed at the same time by Hund and Mulli- ken [8–13], and served initially as a conceptual framework in spectroscopy. Somewhat later,Molecular Lennard-Jones orbital and (MO) Hückel theory applied was developedMO theory atto theelectronic same time structures by Hund of molecules and Mul- [14liken–17 [].8 –13], and served initially as a conceptual framework in spectroscopy. Some- whatSoon later, enough Lennard-Jones the two theories and Hückel and their applied major MO proponents theory to started electronic a struggle structures to dom- of inatemolecules the conceptual [14–17]. frame of chemistry. Initially, MO theory was not accepted too easily by chemists,Soon enough and until the twothe late theories 1950s, and VB their theory major which proponents used a chemical started a language struggle towas domi- up. Subsequently,nate the conceptual MO theory frame was of chemistry. implemented Initially, in useful MO theorysemi-empirical was not programs, accepted too and easily was graduallyby chemists, popularized and until theby eloquent late 1950s, proponents VB theory like which Coulson used a[18], chemical Dewar language [19], and wasothers. up. TheseSubsequently, developments MO theory and the was consequences implemented of in the useful VB– semi-empiricalMO struggle, on programs, the reputation and was of VBgradually theory, popularized determined byits eloquentgradual proponentsdownfall. However, like Coulson with [18 the], Dewardevelopments [19], and of others. new conceptualThese developments frames and and new the computational consequences methods of the VB–MO during struggle,the 1970s ononwards the reputation, VB theory of beganVB theory, to enjoy determined a renaissance its gradual and reoccupy downfall. its place However, alongside with MO the theory developments and DFT. of new conceptual frames and new computational methods during the 1970s onwards, VB theory 2.began The toRoots enjoy and a renaissance Development and of reoccupy VB Theory its place alongside MO theory and DFT. 2. TheLewi Rootss’ formulation and Development of the nature of VB of Theory the chemical bond is in many ways the precursor of VB theory. Lewis’ paper, “The Atom and The Molecule” [1], contains plenty of ideas, some Lewis’ formulation of the nature of the chemical bond is in many ways the precursor of of which were later embedded into VB theory. His electron-pair model and its dynamic VB theory. Lewis’ paper, “The Atom and The Molecule”[1], contains plenty of ideas, some of nature regarding polarity, was formulated 11 years before the onset of VB theory in phys- which were later embedded into VB theory. His electron-pair model and its dynamic nature ics [2]. Moreover, his work was portable and instrumental for the emergence of the new regarding polarity, was formulated 11 years before the onset of VB theory in physics [2]. VB theory of bonding in the hands of Pauling and Slater [2]. It is appropriate, therefore, Moreover, his work was portable and instrumental for the emergence of the new VB theory to begin this section by briefly describing Lewis’ contribution to the nature of the chemical of bonding in the hands of Pauling and Slater [2]. It is appropriate, therefore, to begin this bond.section by briefly describing Lewis’ contribution to the nature of the chemical bond. 2.1. The The Lewis Lewis Electronic Electronic Cubes and Electron Pair Bonds While Lewis f formulatedormulated the electron electron-pair-pair bonds, he also tried to relate these bonds toto the experience of the practicing chemists in the communities of inorganic and organic chemistries [1,20]. [1,20]. In In broad broad terms, terms, the the inorganic chemists were observing chemistry of charged s speciespecies (ions in today’s language), or molecules undergoing dissociation to ions (like acids), while the organic chemists did did not observe much much of of an ionic chemistry, and were interested in the “structure” of their compounds [[2,21,22]2,21,22].. The term “structure” waswas abstract in those early days (mid 19th19th century), and its representations lumped together groups of atom (which appear in many many molecules) molecules) and were were odd odd-looking-looking in modern terms (e.g.(e.g.,, the initial sausages model of benzene by KekulKekuléé [[21,23]21,23]).). Nevertheless, this was the chemical landscape which Lewis tried to describe in terms of bonds between atoms. The first first model model in in the the key key Lewis Lewis paper paper [1] [1 is] isitself itself quite quite cumbersome cumbersome and and is based is based on hison hisrepresentation representation of the of atom the atom as a ascube a cube with witha valence a valence-shell-shell that may that cont mayain contain up to upeight to electronseight electrons (later (laterto be tocalled be called the Octet the Octet Rule) Rule) placed placed at the at corners the corners of the of thecube cube—a—a model model he hadhe had developed developed already already in 1902 in 1902 [24]. [Figure24]. Figure 1 describes1 describes how howLewis Lewis viewed viewed the three the threeputa- tiveputative states states of a single of a single bond bondin the in molecule the molecule like dihalogen like dihalogen underunder different different conditions conditions using theusing cubic the model. cubic model. His view His of view the bond of the is bond dynamic is dynamic and he andis aware he is that aware a bond that can a bond change can itschange character its character in different in different environment environmentss and depending and depending on the nature on the of nature the atoms.