Kinetics and mechanisms of the oxidation of hypophosphite and phosphite with trans-[Ru-VI(L)(O)(2)](2+) (L=1,12-dimethyl-3,4 : 9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane)

The kinetics of the oxidation of hypophosphite and phosphite by trans-[Ru(L )(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclop entadecane) have been studied in aqueous acidic solutions. The reactions ha ve the following stoichiometry (x = 2 or 3): trans-[Ru-VI(L)(O)(2)](2+) + H 2POx- + H2O --> trans-[Ru-IV(L)(O)(OH2)](2+) + H2POx+1-. The two reactions have the same rate law (P = hypophosphite or phosphite): -d[Ru-VI]/dt = k/( 1 + [H+]/K)[Ru-VI][P]. For hypophosphite, k = (1.3 +/- 0.1) dm(3) mol(-1) s (-1) and K = (9.7 +/- 0.5) x 10(-2) mol dm(-3) at 298 K and I = 1.0 mol dm( -3). For phosphite, k = (4.8 +/- 0.4) x 10(-2) dm(3) mol(-1) s(-1) and K = (1.2 +/- 0.2) x 10(-2) mol dm(-3) at 298 K and I = 0.2 mol dm(-3). The effe cts of temperature were studied from 15 degrees C to 40 degrees C. For hypo phosphite, Delta H double dagger = (60 +/- 2) kJ mol(-1) and Delta S double dagger = (-41 +/- 4) J mol(-1) K-1 at pH = 1.86 and I = 1.0 mol dm(-3). Fo r phosphite, Delta H double dagger = (59 +/- 4) kJ mol(-1) and Delta S doub le dagger = (-75 +/- 13) J K-1 mol(-1) at pH = 2.3 and I = 0.2 mol dm(-3). Deuterium isotope effects have also been investigated. For hypophosphite, t he kinetic isotope effect, k(H2PO2-)/k(D2PO2-) is 4.1 at pH = 1.07 and I = 1.0 mol dm(-3). For phosphite, the kinetic isotopic effect, k(HDPO3-)/k(D2P O3-), is 4.0 at pH = 2.30 at I = 0.2 mol dm(-3). A mechanism involving hydr ide transfer from P-H to Ru=O is proposed for these two reactions.