A Brief Story of Valence Bond Theory, its Rivalry with Molecular Orbital Theory, its Demise, and Resurgence

Chapter 1 recounts the evolution of valence bond and molecular orbital theories (VBT and MOT) in chemistry, by interactions between experimental facts and theoretical predictions, which led to a slow process of understanding that the two theories are ultimately identical, but their identity is guised behind different formulations. Thus, VBT starts by describing the electronic structure from atomic or hybrid orbitals, whereas MOT begins from the delocalized MOs. The chapter provides historical backgrounds for these theories: VBT, which describes the electronic structures of molecules as linear combinations of polyelectron VB structures, and MOT, which describes, e.g., O 2 and benzene, using delocalized one-electron MOs. The chapter introduces the key contributions to VBT by, e.g., Lewis, Heitler, London, Pauling, and the present book authors, and to MOT by, e.g., Mulliken, Hund, Slater, Lennard-Jones, and Hoffmann. Ultimately, it is argued that VBT and MOT are equivalent representations in chemistry.