Structure of the hydrated platinum(II) ion and the cis-diammineplatinum(II) complex in acidic aqueous solution:: An EXAFS study

Careful analysis of Pt L-3-edge extended X-ray absorption fine structure (EXAFS) spectra shows that the hydrated platinum(II) ion in acidic (HCIO4) aqueous solution binds four water molecules with the Pt-O bond distance 2.01(2) angstrom and one (or two) in the axial position at 2.39(2) angstrom. The weak axial water coordination is in accordance with the unexpectedly small activation volume previously reported for water exchange in an interchange mechanism with associative character. The hydrated cis-diammineplatinum(II) complex has a similar coordination environment with two ammine and two aqua ligands strongly bound with Pt-O/N bond distances of 2.01(2) angstrom and, in addition, one (or two) axial water molecule at 2.37(2) angstrom. This result provides a new basis for theoretical computational studies aiming to connect the function of the anticancer drug cis-platin to its ligand exchange reactions, where usually four-coordinated square planar platinum(II) species are considered as the reactant and product. Pt-195 NMR spectroscopy has been used to characterize the Pt(II) complexes.