Geometry and hydration structure of Pt(II) square planar complexes [Pt(H2O)4]2+ and [PtCl4]2- as studied by X-ray absorption spectroscopies and quantum-mechanical computations

The geometrical structure of the tetraaquo and tetrachloro Pt(II) complexes in aqueous solutions has been studied by means of X-ray absorption spectroscopies (EXAFS and XANES), combined with quantum-mechanical computations. The latter were carried out to supply independent information about the arrangement of water molecules around the complexes. To this aim the [PtCl4](2-). (H2O)(2) and [Pt(H2O)(4)](2+). (H2O)(8) structures were optimized and the XANES spectra computed using this theoretical structural information were compared with the experimental spectra. From this comparison it was deduced that the hydration shell of the tetraaquo complex was responsible for a small feature of the XANES spectrum above the white line. Pt-Cl distance in [PtCl4](2-) units, both in the crystalline compound, K2PtCl4, and in aqueous solution, was found to be 2.30 Angstrom. Pt-O distance in [Pt(H2O)(4)](2+) species was 2.02 Angstrom. No evidence of stable axial water molecules was found in the aquo complex case. Quantum-mechanical optimization of [PtCl4](2-). (H2O)(2) aggregate indicated that,vater molecules adopt axial orientation with a Pt-O distance of 3.3 Angstrom.