Chapter 3: Electronic Structure of Metal Complexes

This chapter describes crystal field theory and ligand field theory, which are the most fundamental concepts for understanding the electronic structures of transition metal complexes. Both of these theories are essential for a systematic understanding of the correlation between the structure and electronic state of transition metal complexes, and are the basis for learning about the magnetism and electronic absorption spectra of metal complexes. In crystal field theory, all ligands are treated as point charges (or electric dipole moments), and their electrostatic interaction with a d-electron in the metal centre is calculated using a perturbation method, leading to the crystal field splitting of the d orbitals. In ligand field theory, the interactions of σ- and π-orbitals of the ligands with the valence orbitals of the metal are estimated with LCAO molecular orbital techniques. In both cases, symmetry techniques based on group theory are used. The angular overlapping model is described briefly, as a semi-quantitative orbital quantum technique for estimating the splitting energies of the d orbitals from σ- and π-donor ligands. Although it is not widely used at present, treatment using the valence bond method is also outlined and its relationship to crystal field theory and ligand field theory is explained.