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.