13968-48-4Relevant articles and documents
Reactivity of the yl -bond in uranyl(VI) complexes. 1. Rates and mechanisms for the exchange between the trans-dioxo oxygen atoms in (UO 2)2(OH)22+ and mononuclear UO 2(OH)n2-n complexes with solvent water
Szabo, Zoltan,Grenthe, Ingmar
, p. 9372 - 9378 (2007)
The stoichiometric mechanism, rate constant, and activation parameters for the exchange of the yl -oxygen atoms in the dioxo uranium(VI) ion with solvent water have been studied using 17O NMR spectroscopy. The experimental rate equation, v→ = k2obs[UO 22+]tot2/[H+] 2, is consistent with a mechanism where the first step is a rapid equilibrium 2U17O22+ + 2H2O ? (U17O2)2(OH)22+ + 2H+, followed by the rate-determining step (U17O 2)2(OH)22+ + H2O ? (UO2)2(OH)22+ + H2 17O, where the back reaction can be neglected because the 17O enrichment in the water is much lower than in the uranyl ion. This mechanism results in the following rate equation v→ = d[(UO2)2(OH)22+]/dt = k 2,2[(UO2)2(OH)22+] = k2,2*β2,2[UO22+] 2/[H+]2; with k2,2 = (1.88 ± 0.22) ± 104 h-1, corresponding to a half-life of 0.13 s, and the activation parameters ΔH? = 119 ± 13 kJ mol-1 and ΔH? = 81 ± 44 J mol-1 K-1. *β2,2 is the equilibrium constant for the reaction 2UO22+ + 2H2O ? (UO2)2(OH)22+ + 2H +. The experimental data show that there is no measurable exchange of the yl -oxygen in UO22+, UO 2(OH)+, and UO2(OH)42-/ UO2(OH)53-, indicating that yl -exchange only takes place in polynuclear hydroxide complexes. There is no yl -exchange in the ternary complex (UO2) 2(μ-OH)2(F)2(oxalate)2 4-, indicating that it is also necessary to have coordinated water in the first coordination sphere of the binuclear complex, for exchange to take place. The very large increase in lability of the yl -bonds in (UO2)2(OH)22+ as compared to those of the other species is presumably a result of proton transfer from coordinated water to the yl -oxygen, followed by a rapid exchange of the resulting OH group with the water solvent. Yl -exchange through photochemical mediation is well-known for the uranyl(VI) aquo ion. We noted that there was no photochemical exchange in UO2(CO3) 34-, whereas there was a slow exchange or photo reduction in the UO2(OH)42- / UO2(OH) 53- system that eventually led to the appearance of a black precipitate, presumably UO2.
Redox chemistry of the acetato-bridged clusters [M3( 3-O)n(-O2CCH3)6(H 2O)3]2+ (M = Mo, W, n = 1, 2): Reversible redox between mono-3-oxo d8 MIII2M IV and d9 MIII3 forms
Powell, Glenmore,Richens, David T.
, p. 2959 - 2963 (2006)
A cyclic voltammogram of aqueous 0.1 mol dm-3 triflic acid solutions of the d6 bioxo-capped M-M bonded cluster [Mo 3(3-O)2(O2CCH3) 6(H2O)3]2+ at a glassy carbon electrode at 25 °C gives rise to an irreversible 3e- cathodic wave to a d9 MoIII3 species at -0.8 V vs. SCE which on the return scan gives rise to two anodic waves at +0.05 V vs. SCE (E1/2, 1e- reversible to d8 Mo III2MoIV) and +0.48 V vs. SCE (2e- irreversible back to d6 MoIV3). The number of electrons passed at each redox wave has been confirmed by redox titration and controlled potential electrolysis which resulted in 90% recovery of [Mo 3(3-O)2(O2CCH3) 6(H2O)3]2+ following electrochemical re-oxidation at +0.8 V. A corresponding CV study of the d8 monoxo-capped WIII2WIV cluster [W 3(3-O)(O2CCH3)6(H 2O)3]2+ gives rise to a reversible 1e - cathodic process at -0.92 V vs. SCE to give the d9 WIII3 species [W3(3-O)(O 2CCH3)6(H2O)3] +; the first authentic example of a WIII complex with coordinated water ligands. However the cluster is too unstable (O 2/water sensitive) to allow isolation. Comparisons with the cv study on [Mo3(3-O)2(O2CCH 3)6(H2O)3]2+ suggest irreversible reduction of this complex to monoxo-capped [MoIII 3(3-O)(O2CCH3)6(H 2O)3]+ followed by reversible oxidation to its d8 counterpart [Mo3(3-O)(O2CCH 3)6(H2O)3]2+ (Mo III2MoIV) and finally irreversible oxidation back to the starting bioxo-capped cluster. Exposing the d9 Mo III3 cluster to air (O2) however gives a different final product with evidence of break up of the acetate bridged framework. Corresponding redox processes on d6 [W3( 3-O)2(O2CCH3)6(H 2O)3]2+ are too cathodic to allow similar generation of the monoxo-capped WIII3 and W III2WIV clusters at the electrode surface. The Royal Society of Chemistry 2006.
The slowest water exchange at a homoleptic mononuclear metal center: Variable-temperature and variable-pressure 17O NMR study on [Ir(H2O)6]3+
Cusanelli, Antonio,Frey, Urban,Richens, David T.,Merbach, André E.
, p. 5265 - 5271 (2007/10/03)
The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua iridium(III) have been determined by 17O NMR spectroscopy as a function of temperature (358-406 K) and pressure (0.1-210 MPa) at several acidities (0.5-5.0 m). Noncoordinating trifluoromethanesulfonate (CF3SO3-) was used as the counterion. The observed rate constant was of the form k = k1 + k2/[H+], where the subscripts 1 and 2 refer to the exchange pathways on [Ir(H2O)6]3+ and [Ir(H2O)5(OH)]2+, respectively. The kinetic parameters obtained are summarized as follows: k1298 = (1.1 ± 0.1) × 10-10 s-1, ΔH1? = 130.5 ± 0.6 kJ mol-1, ΔS1? = +2.1 ± 1.7 J K-1 mol-1, and ΔV1? = -5.7 ± 0.5 cm3 mol-1; k2298 = (1.4 ± 0.6) × 10-11 m s-1, ΔH2? = 138.5 ± 4.5 kJ mol-1, ΔS2? = +11.5 ± 11.6 J K-1 mol-1, and ΔV2? = -0.2 ± 0.8 cm3 mol-1. The value obtained for k1298 corresponds to a residence time of ca. 300 years. The pKa298 and the volume change ΔVa0 associated with the first hydrolysis of [Ir(H2O)6]3+ were determined by potentiometric and high-pressure spectrophotometric methods to be 4.45 ± 0.03 and -1.5 ± 0.3 cm3 mol-1, respectively. Utilizing the relation k2 = kOHKal, values for the first-order rate constant and the corresponding activation volume for [Ir(H2O)5(OH)]2+ were estimated to be kOH298 = 5.6 × 10-7 s-1 and ΔVOH? = +1.3 cm3 mol-1, respectively. These data are supportive of an associative interchange (Ia) mechanism for water exchange on [Ir(H2O)6]3+, but of an interchange (I) mechanism on the deprotonated species [Ir(H2O)5(OH)]2+. These mechanistic results have also been compared to those reported for other trivalent metal ions.
