Kinetics and mechanism of the oxidation of the hypophosphite ion by the tetrahydroxoargentate(III) ion

Abstract

The kinetics of oxidation of hypophosphite ion to phosphite by the tetrahydroxoargentate(III) ion, Ag(OH)4-, has been studied in strong base by stopped-flow spectrophotometry. The measured 1:1 stoichiometry is consistent with a two-electron change, which is equivalent to an oxygen atom transfer from Ag(III) to H2PO2-as in the case of arsenic(III) and sulfite ion oxidations. The reaction is pseudo first order in [silver(III)] disappearance with kobsd= (a + b[OH-])[H2PO2-], where a = 0.069 ± 0.014 dm3mol-1s-1and b = 1.13 ± 0.02 dm6mol-2s-1at 25 °C and μ = 1.58 mol dm-3. Reaction with D2PO2-is considerably slower (aH/aD = 2.46; bH/bD= 8.2), indicating that P-H bond breaking is the rate-determining step. The activation parameters for the [OH-]-independent and -dependent paths are ΔHa*= 82 ± 10 kJ mol-1, ΔSa* = 10 ± 32 J K-1mol-1and ΔHb* = 39 ± 5 kJ mol-1, ΔSb* = -114 ± 16 J K-1mol-1, respectively. The ionic strength dependence of the rate reflects competition between the second-and third-order paths. The proposed mechanism involves the formation of an intermediate between an axial site on Ag(OH)4-and H2PO2-. The intermediate undergoes redox by two modes—both internal and OH-assisted—which involve the deprotonation of the P-H bond in the rate-determining steps. Comparison with results of other oxoanion oxidations by silver(III) suggests a common mechanism involving oxygen atom transfer from Ag(OH)4-to an axially bound reductant. © 1989, American Chemical Society. All rights reserved.