Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation

The local and geometrical structure around gold (III) e.g., AU(3+) ions in aqueous solution with different OH-/Cl- molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCln(OH)(4-n)](-) solutions have been calculated and the multiple-scattering ectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl- ligands by OH- ligands. A square-planar geometry for [AuCln(OH)(4-n)](-) with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 angstrom for Au-Cl and 1.98 angstrom for Au-O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 angstrom in solutions with the lowest OH-/Cl- molar ratio arises from collinear Cl-Au-Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.