Levels of VB and MO Theories

Abstract

Given the present preeminence of MO theory, qualitative as well as quantitative, there is an unavoidable tendency to forget how frequently we actually revert to VB concepts when we discuss complicated systems or attempt to isolate the physical origin of a chemical phenomenon. Thus, the experimentalist still finds “arrow pushing” a useful tool in devising new synthetic routes or formulating mechanisms involving large molecules. Even more telling is the fact that in exploring the electronic nature of excited states and chemical intermediates one realizes the conceptual convenience of “radicaloid” and “zwitterionic” states.^35a In attempting to understand how CI modifies the monodeterminantal MO wavefunction, one begins to appreciate the precarious balance of “covalent” and “ionic” terms.⁷¹ Finally, even in ground state stereochemical problems, the appeal of the “bond” construct is hard to resist and many papers have been published which make intuitive use of VB theoretical methodology.⁷² The purpose of this two-Dart work is to tailor VB theory into a potent intelligible, and formally correct qualitative theory of chemical bonding which can be applied uniformly to problems in all subdisciplines of chemistry. That this development is a natural and unavoidable occurence within the framework of scientific evolution can only be appreciated if one has a clear understanding of diverse theoretical approaches and their interrelationships, hence, this section is devoted to a discussion of the various brands of MO and VB theories which exposes common denominators and fundamental differences.