X-RAY SPECTROSCOPIC STUDIES OF NICKEL-COMPLEXES, WITH APPLICATION TO THE STRUCTURE OF NICKEL SITES IN HYDROGENASES

X-ray absorption near-edge spectra (XANES) are reported for 44 Ni(II) and Ni(III) complexes with N- and/or S-donor ligands. The spectra reveal features associated with 1s --> 3d and 1s --> 4p(z) electronic transitions, whose presence or absence and intensity provide information that allows the coordination number/geometry of the complex to be determined in most cases. The complexes in this study were selected in order to examine the reliability of coordination number/geometry assignments in complexes with low symmetry and to examine the effects on the spectra of a change in formal oxidation state from +II to +III. The effects on the spectra due to changes in the ligand environment are examined, and the edge energy and the breadth of the edge are found to correlate with the average hardness of the ligand environment. The effects on the spectra due to oxidation state changes are examined by using several pairs of Ni(II/III) isoleptic complexes. These compounds reveal that the effects of changes in the formal oxidation state of the Ni center are strongly dependent on the nature of the ligands present, with S-donor ligands giving rise to smaller shifts in edge energy than N,O-donor ligands. These trends are indicative of the increasing role of ligand oxidation in Ni(III) thiolate complexes. These trends are corroborated by X-ray photoelectron spectroscopic (XPS) studies that show a similar trend in both ligand and metal electron binding energies. The information obtained from the model studies is used to examine the Ni K-edge XANES spectrum obtained from a sample of Thiocapsa roseopersicina hydrogenase poised in form C. This spectrum is shown to be consistent with a distorted trigonal-bipyramidal geometry and a mixed O,N- and S-donor ligand environment for this biological Ni site.