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 Hg2+aq- and HgCl+aq-assisted aquations of [PtCl4]2- (1), [PtCl3(H2O)]- (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)2Me2SO]+ (8), trans-[PtCl(H2O)2Me2SO]+ (9), trans-[PtCl2(NH3)2] (10), and cis-[PtCl2(NH3)2] (11) have been studied at 25.0 °C in a 1.00 M HClO4 medium buffered with chloride, using stopped-flow and conventional spectrophotometry. Saturation kinetics and instantaneous, large UV/vis 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 kinetics data together with the UV/vis spectral changes and DFT calculations indicate that their structures are characterized by axial coordination of Hg to Pt with remarkably short metal−metal bonds. Stability constants for the Hg2+ adducts with complexes 1−6, 10, and 11 are (2.1 ± 0.4) × 104, (8 ± 1) × 102, 94 ± 6, 13 ± 2, 5 ± 2, 60 ± 6, 387 ± 2, and 190 ± 3 M-1, respectively, whereas adduct formation with the sulfoxide complexes 7−9 is too weak to be observed. For analogous platinum(II) complexes, the stabilities of the Pt−Hg adducts increase in the order sulfoxide ≪ aqua < ammine complex, reflecting a sensitivity to the π-acid strength 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 contributions for Hg2+ and HgCl+ reveal that the latter does not form dinuclear adducts of any significant stability. The overall experimental evidence is consistent with a mechanism in which the accumulated Pt(II)−Hg2+ adducts are not reactive intermediates along the reaction coordinate. The aquation process occurs via weaker Pt−Cl−Hg or Pt−Cl−HgCl bridged complexes.