Geometry and hydration structure of Pt(II) square planar complexes [Pt(H2O)(4)](2+) and [PtCl4](2-) as studied by X-ray absorption spectroscopies andquantum-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 spectro scopies (EXAFS and XANES), combined with quantum-mechanical computations. T he latter were carried out to supply independent information about the arra ngement 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 th e XANES spectra computed using this theoretical structural information were compared with the experimental spectra. From this comparison it was deduce d that the hydration shell of the tetraaquo complex was responsible for a s mall feature of the XANES spectrum above the white line. Pt-Cl distance in [PtCl4](2-) units, both in the crystalline compound, K2PtCl4, and in aqueou s 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 wa s found in the aquo complex case. Quantum-mechanical optimization of [PtCl4 ](2-). (H2O)(2) aggregate indicated that,vater molecules adopt axial orient ation with a Pt-O distance of 3.3 Angstrom.