Myth and reality in the attitude toward valence-bond (VB) theory: Are its 'failures' real?

According to common wisdom propagated in textbooks and papers, valence-bond (VB) theory fails and makes predictions in contradiction with experiment. Four iconic 'failures' are: a) the wrong prediction of the ground state of the O₂ molecule, b) the failure to predict the properties of cyclobutadiene (CBD) viz. those of benzene, c) the failure to predict the aromaticity/anti-aromaticity of molecular ions like C₅H₅⁺ and C₅H₅⁺, C₃H₃⁺ and C₃H₃⁺, C₇H₇⁺ and C₇H₇^-, etc; and d) the failure to predict that, e.g., CH₄ has two different ionization potentials. This paper analyzes the origins of these 'failures' and shows that two of them (stated in a and d) are myths of unclear origins, while the other two originate in misuse of an oversimplified version of VB theory, i.e., simple resonance theory that merely enumerate resonance structures. It is demonstrated that, in each case, a properly used VB theory at a simple and portable level leads to correct predictions, as successful as those made by use of molecular-orbital (MO) theory. This notion of VB 'failure', which is traced back to the VB-MO rivalry, in the early days of quantum chemistry, should now be considered obsolete, unwarranted, and counter-productive. A modern chemist should know that there are two ways of describing electronic structure, which are not two contrasting theories, but rather two representations or two guises of the same reality. Their capabilities and insights into chemical problems are complementary, and the exclusion of any one of them undermines the intellectual heritage of chemistry.