Kinetics and mechanism for reversible chloride transfer between mercury(II) and square-planar platinum(II) chloro ammine, aqua, and sulfoxide complexes. Stabilities, spectra, and reactivities of transient metal-metal bonded platinum-mercury adducts

The Hg-aq(2+)- and HgClaq+-assisted aquations of [PtCl4](2-) (1), [PtCl3(H2 O)](-) (2), cis-[PtCl2(H2O)(2)] (3), trans-[PtCl2(H2O)(2)] (4), [PtCl(H2O)( 3)](+) (5), [PtCl3Me2SO](-) (6), trans-[PtCl2(H2O)Me2SO] (7), cis-[PtCl(H2O )(2)Me2SO](+) (8), trans-[PtCl(H2O)(2)Me2SO](+) (9), trans-[PtCl2(NH3)(2)] (10), and cis-[PtCl2(NH3)(2)] (11) have been studied at 25.0 degrees C in a 1.00 M HClO4 medium buffered with chloride, using stopped-flow and convent ional spectrophotometry. Saturation kinetics and instantaneous, large UV/vi s spectral changes on mixing solutions of platinum complex and mercury are ascribed to formation of transient adducts between Hg2+ and several of the platinum complexes. Depending on the limiting rate constants, these adducts are observed for a few milliseconds to a few minutes. Thermodynamic and ki netics data together with the UV/vis spectral changes and DFT calculations indicate that their structures are characterized by axial coordination of H g to Pt with remarkably short metal-metal bonds. Stability constants for th e Hg2+ adducts with complexes 1-6, 10, and 11 are (2.1 +/- 0.4) x 10(4), (8 +/- 1) x 10(2), 94 +/- 6, 13 +/- 2, 5 +/- 2, 60 +/- 6, 387 +/- 2, and 190 +/- 3 M-1, respectively, whereas adduct formation with the sulfoxide comple xes 7-9 is too weak to be observed. For analogous platinum(II) complexes, t he stabilities of the Pt-Hg adducts increase in the order sulfoxide much le ss than aqua < ammine complex, reflecting a sensitivity to the pi-acid stre ngth of the Pt ligands. Rate constants for chloride transfer from HgCl+ and HgCl2 to complexes 1-11 have been determined. Second-order rate constants for activation by Hg2+ are practically the same as those for activation by HgCl+ for each of the platinum complexes studied, yet resolved contribution s for Hg2+ and HgCl+ reveal that the latter does not form dinuclear adducts of any significant stability. The overall experimental evidence is consist ent with a mechanism in which the accumulated Pt(II)-Hg2+ adducts are not r eactive intermediates along the reaction coordinate. The aquation process o ccurs via weaker Pt-Cl-Hg or Pt-Cl-HgCl bridged complexes.