Ligand K-edge X-ray Absorption Spectroscopic Studies Of-the Electronic Structure of Inorganic Model Complexes and Metalloprotein Active Sites

Ligand K-edge X-ray absorption spectroscopy (XAS) has been developed as a technique for the investigation of l&and-metal bonding and has been applied to the study of electronic structure in inorganic model complexes and metalloprotein active sites. Bonding in Cl-Cu(I1) Complexes. Ligand K-edge XAS has been measured at the chloride K-edge for a series of complexes containing chloride ligands bound to open shell dg copper ions. The intensity of the pre-edge feature in these spectra reflects the covalency in the half-occupied d,2-y2-derived molecular orbital (HOMO) of the complex. The energy of the pre-edge feature is related to both the charge on the ligand and the HOMO energy. An analysis of the intensity and energy of the pre-edge feature as well as the energy of the rising edge absorption provides quantitative information about the covalency of the ligand-metal interaction, the charge donated by the chloride, and the energy of the copper d-manifold. The results demonstrate that ligand K-edge XAS features can be used to obtain quantitative information about ligand-metal bonding. The results also identify the chemical basis for trends in the XAS data for the complexes: D4h CLICKS-, D2d CUCKOO-, planar, trans-CuC12(pdmp)2 (pdmp=N-phenyl-3,5-dimethylpyrazole), square pyramidal CuCl53-, the planar dimer KCuCl3, the distorted tetrahedral dimer (Ph4P)CuC13, and two dimers with mixed ligation, one containing a bridging chloride, and the other, terminally bound chloride. A geometric distortion from square planar to distorted tetrahedral results in a decrease in the chloride-copper HOMO covalency but an increase in the total charge donation by the chlorides. Thus, while the geometry can maximize the overlap for a highly covalent HOMO, this does not necessarily reflect the overall charge donation. The Cl-Cu(II) bonding interactions are dependent on the nature of the other coordinating ligands. Replacement of chlorides by less strongly donating ligands causes an increase in charge donation by the remaining chloride ligands. An increase in the coordination number of the copper or in the charge donation by the ligands (resulting in a lower effective nuclear charge on the copper) causes an increase in the copper d-manifold energy. Finally, the bonding of a terminal vs. bridging chloride is very different, in that for the latter there is more total charge donation and a higher ligand covalent contribution to the HOMO orbital of the two coppers. Investigation of the Source of the Small EPR All Splitting in Blue Copper. XAS for the oxidized blue copper protein plastocyanin and several Cu(II) model complexes have been measured at both the Cu K-edge and the ligand (Cl and S) K-edges in order to elucidate the source of the small parallel hyperfine splitting in the EPR spectra of blue copper centers. A feature in the Cu K-edge X-ray absorption