Ligand field and molecular orbital theories of transition metal X-ray absorption edge transitions

Carl Ballhausen made a wide range of seminal contributions to ligand field theory and its application to ground state and ligand field excited state spectroscopies. These provided a fundamental basis for probing the nature of transition metal complexes using their visible spectra and a range of magnetic spectroscopies. The advent of synchrotrons provided access to high flux electromagnetic radiation that could be tuned across a wide range of energies including X-ray. This expanded the scope of spectroscopic techniques available to inculde X-ray Absorption Edge Spectroscopies. Paralleling a visible absorption experiment, X-ray spectra (metal K-edge, i.e. 1s→3d and metal L-edge, i.e. 2p→3d) taken at a synchrotron are dominated by ligand field splittings, electron repulsion effects and covalency. These can be used to obtain important insight into the properties of a diverse range of materials from solar cells to the catalytic centers of metalloenzymes. Herein we systematically consider applications of ligand field theory to X-ray absorption edge transitions