Mixing s orbitals into p and d orbitals. An attempt at bridging the angular overlap model and the valence shell electron pair repulsion model. Critique of the cellular ligand-field model

Abstract

The stereochemistries of main group molecules have been discussed by using the angular overlap model in its molecular orbital oriented form (MO-AOM). Either ligand-field stabilisation of the ground state s 2p q−2 configuration, or s-p mixing, or both, provide a consistent bonding model for the stereochemistries. The transformation of the non-bonding orbitals into equivalent orbitals leads invariably to agreement with the lone-pair locations of the valence shell electron pair repulsion (VSEPR) model. The concepts of Hamiltonian-generated hybrids and pseudohemispherical molecular systems are found useful in this context. The MO-AOM formalism is also used for discussing s-d mixing in transition metal systems, and the energetic consequences within the ligand-field AOM (LF-AOM) are included. This is a second-order effect, which depends on squares and cross-products of radial parameters. It may still be quite large for tetragonal systems and for systems that deviate strongly from orthoaxiality. The usual ligand-additive property of the AOM is lost when the symmetry is lower than tetragonal and so is the energy separability into angular and radial factors. The cellular ligand-field model is found to be identical with the LF-AOM, except that its users consider it important not to acknowledge the formal hierarchy, MO-AOM⊃LF-AOM, as relevant. The unintelligible concept of an active coordination void is found to be unnecessary and insufficient.