MOVB Theory

Abstract

The most useful physical model which can form the basis for a theoretical analysis of a chemical problem is the “Fragments in Molecules” (FIM) model. Variants of the FIM model include the “Atoms in Molecules” (AIM),¹⁸ “Diatomics in Molecules” (DIM),¹⁹ and “Molecules in Molecules” (MIM)²⁰ models. In principle, these can be used in conjunction with either MO or VB theory. The MOVB theory of chemical bonding we are about to develop makes use of a different type-of FIM model, namely, the Core-Ligand (CL) model which projects how bonds are formed between an atomic or molecular core and a set of ligands. For example, we shall view methane as the product of the union of C and H₄, water as the result of combining O and H₂ etc. Each of the two fragments has an associated orbital manifold and configurations can be built by distributing all valence electrons among all valence orbitals of the two fragments in all possible ways with each MOVB CW written as a Slater determinant or a linear combination of Slater determinants. Now, in Part I, we stated that one of the disadvantages of MO theoretical models is the absence of a unique or universally acceptable reference frame. This difficulty is removed at the level of VB theory where the free atoms constitute an unambiguous frame of reference. By contrast, the problem persists in any formalism which contracts an AO basis to an MO basis, i.e., it persists at the level of MOVB theory. The great advantage of the CL model is that it copes with the “frame of reference difficulty” in the best possible way because every conceivable chemical system can be formulated as a composite of a central core and surrounding ligands, a reference electronic configuration can be unambiguously and universally defined (vide infra), and the bonds linking the core and the ligands can be described correctly by the MOVB method to be used in connection with this model.