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

Abstract

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)2M32SO]+ (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 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) 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 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 pi-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.

A Thermally Stable Pt/Y-Based Metal−Organic Framework: Exploring the Accessibility of the Metal Centers with Spectroscopic Methods Using H2O, CH3OH, and CH3CN as Probes

“…49 The edge in the visible region (at 19 100 cm -1 ) can be associated to Cl to Pt ligand-to-metal charge transfer (LMCT). 50 Electronic transitions related to Y are expected at higher energies, where the spectra have not been reported owing to a too poor S/N ratio.…”

“…The UV−vis spectrum of as-prepared Pt/Y-MOF sample (bold black curve in Figure a) is dominated by a band centered at 29 200 cm -1 , caused by the intense π → π* transition of the aromatic rings. , A relatively broad band located at 25 300 cm -1 can be assigned to metal-to-ligand charge transfer (MLCT) from Pt to bipyridine . The edge in the visible region (at 19 100 cm -1 ) can be associated to Cl to Pt ligand-to-metal charge transfer (LMCT) . Electronic transitions related to Y are expected at higher energies, where the spectra have not been reported owing to a too poor S/N ratio.…”

Kinetics and Mechanism of Ligand Substitution in Some Chromium(III) Tetraaza Macrocyclic Complexes in Acidic Media: Differences in Reactivity Between cis and trans Isomers

“…Examples are known, however, of mercury(II) causing rate retardation by coordination at another available position and not at the ligand which is the leaving group. [15] The mercury(II)-induced aquation reactions of the present macrocyclic complexes have been studied in the presence of an excess of mercury(II) sufficient to transform the aquathiocyanatochromium(III) reagents into [Cr III -NCSHg II ]-type complexes with a degree of conversion exceeding 95 %. This guarantees independence of the observed rate constant on the mercury(II) concentration.…”

The hydrated platinum(ii) ion in aqueous solution—a combined theoretical and EXAFS spectroscopic study

“…The presence of weakly bound water molecules, with very fast exchange rate in the axial positions is the basis for the interpretation of the kinetics of substitution reactions of metal ions binding four solvent molecules in square-planar fashion. 12, 13 Neither palladium(II) nor platinum(II) ions crystallize as fully hydrated ions, the structure determination must be performed in acidified aqueous solution to avoid hydrolysis; the pK a values of the hydrated palladium(II) and platinum(II) ions are reported to be 3.0 ± 0.1 and >2.5, respectively. 14,15 Experimental Chemicals A 15 mmol dm -3 aqueous solution of platinum(II) perchlorate for the EXAFS measurements, containing ca.…”