Ligand K-edge X-ray absorption spectroscopy: A direct probe of ligand-metal covalency

Ligand K-edge X-ray absorption spectroscopy (XAS) is a new experimental probe of the covalency of a metal-ligand bond. The intensity of the ligand pre-edge feature is proportional to the mixing of ligand orbitals into the metal d orbitals. The methodology to determine covalencies in one-electron (hole) and many-electron systems is described and demonstrated for a series of metal tetrachlorides [MCl4](n-), metal tetrathiolates [M(SR)(4)](n-), and dimeric iron-sulfur (Fe-S) clusters [Fe2S2(SR)(4)](2-). It is there applied to blue Cu proteins, the Cu-A site, hydrogen bonding in Fe-S clusters, and the delocalization behavior in [2Fe-2S] vs [4Fe-4S] clusters. The covalencies determined in these studies provide important electronic structure insight into function.