Ligand K-edge X-ray absorption spectroscopy: covalency of ligand–metal bonds

The ligand K-edge probes the ligand 1s → valence np transitions. These transitions acquire intensity when the ligand is bound to an open shell metal ion. This intensity quantifies the amount of ligand character in the metal d orbitals, hence the covalency of the ligand–metal bond. In this review the methodology is developed and applied to copper proteins, iron–sulfur sites and nickel dithiolene complexes, as examples. These illustrate the power and impact of this method in evaluating covalency contributions to electron transfer pathways, reduction potentials, H-bond interactions, electron delocalization in mixed-valent systems and small molecule reactivity.