Molecular orbital theory for inorganic molecules

During the last decade, the concern of theoretical chemists has rested largely with approximate calculations on organic molecules containing n-electrons on the one hand, and with ab initio calculations on very small molecules on the other. As a result of this work, together with the development of large-scale computers, we are now in a position to extend the vision in the direction of larger, inorganic molecules. In terms of the investigation of experimental properties, the re-examination of intuitive chemical concepts, and the formation of new concepts to describe chemical bonding in a more rigorous way, the possibilities of this development are enormous. There ought to be some important implications for the theoretical chemist with respect to theoretical methods, for the experimental chemist with respect to his understanding and explanations of chemical behaviour, and for our knowledge of and teaching about the chemical bond in general. This thesis represents an examination of some of these possibilities. Initially a critical review of ways and means of carrying out theoretical calculations on inorganic systems is necessary. The conclusions drawn from this review are used to consider two particular problem areas in inorganic chemistry: the nature of the chemical bond in molecules containing elements of the second row of the Periodic Table, especially the sulphur-oxygen bond; and the electronic structure and experimental properties of tetrahedral oxy-anions of the transition metals. On the basis of its success in the areas of n-electron calculations for organic molecules, ab initio calculations for small molecules, and ligand field theory for transition metal complexes, together with the large body of experience available about its use in calculations, the molecular orbital theory is chosen as a framework for the methods developed here. We now consider in detail the needs and requirements of a theoretical approach to inorganic molecules, based on this theory.