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 MCl(4)(n)(-), metal tetrathiolates M(SR)(4)(n)(-), and dimeric iron-sulfur (Fe-S) clusters Fe(2)S(2)(SR)(4)(2-). It is then 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.