United atom models for polyatomic ligands. Part I. Hydrogen derivatives

The electronic structures of the hydrides of Group V and VI are investigated with a single-centre molecular orbital model. This model in the form of empirical United Atom correlation diagrams is appropriate to the description of ligand molecules because it predicts with little difficulty the symmetries and energies of the ligand frontier orbitals. The necessary second-order perturbations are discussed in terms of three effects, the Off-Centre-Atom (OCA) field, configuration interaction and the Anisotropic Potential Destablization (APD). The Lewis basicity of the highest occupied ligand orbital is described in terms of the orbital symmetry, localization, and the extent of the APD effect. The chemical behaviour predicted with these spectroscopic criteria is consistant with the observed co-ordinating ability of these ligands.