KINETICS AND MECHANISM FOR OXIDATION OF DISSOLVED SULFUR-DIOXIDE BY TETRACYANODICHLOROPLATINATE (IV) IN ACIDIC AQUEOUS-SOLUTION AND FOR FORMATION AND O-S LINKAGE ISOMERIZATION OF SULFITOTETRACYANOCHLOROPLATINATE(IV)

Oxidation of SO2.nH2O/HSO3-/SO32- by Pt(CN)4Cl22- has been studied at 25-degrees-C and 1.0 M ionic strength in acidic aqueous solution by use of stopped-flow spectrophotometry. The stoichiometry of the reaction is 1:1 according to Pt(CN)4Cl22- + HSO3 + H2O --> Pt(CN)42- + 2Cl- + HSO4- + 2H+. It is first-order with respect to both [Pt(IV)] and [S(IV)]. The pH-dependence in the region 0.3 < pH < 4.5 is complex and indicates that Pt(CN)4Cl22- is reduced by both HSO3- and SO32- in two parallel reactions with second-order rate constants 1.34-0.5 M-1 s-1 and (4.5 +/- 0.1) x 10(5) M-1 s-1, respectively. The suggested mechanism involves attack by the incoming nucleophiles on coordinated chloride, followed by inner-sphere two-electron transfer to the metal center and formation of Pt(CN)42-and ClSO3H and ClSO3-. Chlorosulfuric acid and chlorosulfate(VI) hydrolyze in rapid subsequent reactions to HSO4-. The kinetics for reaction between Pt(CN)4ClOH2- and HSO3- has also been studied by use of variable-temperature and high-pressure stopped-flow spectrophotometry in an aqueous medium with 0.3 < pH < 2 and as a function of ionic strength 0.325 < I < 1.0 M. This is a three-step process. An oxygen-bonded sulfito complex, Pt(CN)4Cl(OSO2)3-, is formed rapidly by addition of HSO3- to the hydroxo ligand of Pt(CN)4ClOH2-, without breakage of the Pt-O bond. In the second step, this complex undergoes a slower, rate-determining intramolecular linkage isomerization to the thermodynamically more stable S-bonded isomer Pt(CN)4Cl(SO3)3-, which is reduced rapidly to Pt(CN)42- and HSO4- in an inner-sphere two-electron process. Rate constants at 25-degrees-C, DELTAH(double-dagger), DELTAS(double-dagger), and DELTAV(double-dagger) are for the uptake reaction (1.68 +/- 0.05) X 10(-5) M-1 s-1, 44 +/- 3 kJ mol-1, -7.5 +/- 0.4 J K-1 mol-1, and -9.4 +/- 0.4 cm3 mol-1 and for the O-S isomerization 4.5 +/- 0.2 s-1, 74.4 +/- 0.5 kJ mol-1, 16.6 +/- 0.1 J K-1 mol-1, and 5.3 +/- 0.4 cm3 mol-1. The positive entropy and volume of activation for the linkage isomerization indicate that this process takes place through an intramolecular rearrangement via a loosely bound transition state, in which the Pt-O bond is weakened before a new Pt-S bond is formed. In all experiments, the concentration of dissolved molecular oxygen remains constant. This is in agreement with the proposed mechanism, according to which electron transfer is completed within the solvent cage and no radical species are released into the bulk solution, starting an autoxidation of S(IV).