Recent developments in valence theory
In the early 1930s valence theory was confused by a conflict between molecular-orbital and valence-bond approximations. But during the l940s a reconciliation between the two methods was found; and from that time until now much the greater effort has been put into MO calculations. These calculations have been tremendously enriched in the 1960s by the widespread use of electronic computers, and package programmes. A study of the charge distribution calculated for covalent bonds shows that the early picture of a bond as being associated with a build-up of charge between the nuclei is still valid. However, if bonding electrons are drawn into this region, other electrons tend to get forced out of it; recent studies show the importance of these two effects. The early representation of these charge-clouds in terms of hybridization was particularly fruitful in the l940s, but is now recognized as too restrictive. The development of the experimental techniques of photoelectron spectroscopy and x-ray spectroscopy now enables a direct verification to be made of the early ideas of individual molecular orbitals. In this way Mulliken's original theoretical descriptions have been triumphantly justified. Another aspect of valence theory that has developed in recent years is that of the relation between a and it electrons. No longer may the it electrons be treated as if they were independent of the a electrons, but a close relationship exists between them. The account concludes with a description of those situations where the number of valence electrons is either too few (electron-deficient molecules) or too large (electron-rich molecules) to provide the normal complement of two electrons per bond. In each case the simple concept of a bond needs to be modified. Attention is drawn to the remarkable way in which theoretical concepts have recently received experimental verification. INTRODUCTION Valence theory is much older than the fifty years referred to in the title of this Symposium. We should at the very least go back to t86 when Odling, Crum Brown and others started using the horizontal line—-which has been the symbol for a chemical bond ever since; and we should think of Kekule only a few years later trying to use the symbol to indicate some sort of geometrical relationship between the various atoms of a polyatomic molecule: or Butlerov, with his early feeling for chemical structure. Moreover, for these last hundred years, as Mulliken and Van Vleck have shown in their