Reactivity of the yl-bond in Uranyl(VI) complexes. 1. Rates and mechanisms for the exchange between the trans-dioxo oxygen atoms in (UO2)(2)(OH)(2)(2+) and mononuclear UO2(OH)(n)(2-n) complexes with solvent water
2007 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 46, no 22, 9372-9378 p.Article in journal (Refereed) Published
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 O-17 NMR spectroscopy. The experimental rate equation, -v = k(2obs)[ UO22+](tot)(2)/[H+](2), is consistent with a mechanism where the first step is a rapid equilibrium 2U(17) O-2(2+) + 2H(2)O reversible arrow ((UO2)-O-17)(2)(OH)(2)(2+) + 2H(+), followed by the rate-determining step ((UO2)-O-17)(2)(OH)(2)(2+) + H2O reversible arrow (UO2)(2)(OH)(2) (2+) + H-2 170, where the back reaction can be neglected because the 170 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)(2)(2+) ]/dt= k(2,2)[(UO2)(2)(OH)(2)(2+)] = k2,2*beta 2.2[ UO22+](2)/[H+]2; with k(2.2) = (1.88 +/- 0.22) x 10(4) h(-1), corresponding to a half-life of 0.13 s, and the activation parameters triangle h4 = 119 +/- 13 kJ mol(-1) and triangle S* = 81 +/- 44 J mol(-1) K-1. *beta 2.2 is the equilibrium constant for the reaction 2UO(2)(2+) + 2H(2)O reversible arrow (UO2)(2)(OH)(2)(2+) + 2H(+). The experimental data show that there is no measurable exchange of the yl-oxygen in UO22+, UO2(OH)(+), and UO2(OH)(4)(2-)/ UO2(OH)(5)(3-), indicating that yl-exchange only takes place in polynuclear hydroxide complexes. There is no yl-exchange in the ternary complex (UO2)(2)(mu-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)(2)(2+) 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 4there was no photochemical exchange in UO2(CO3)(3)(4) whereas there was a slow exchange or photo reduction in the UO2(OH)(4)(2-) / UO2(OH)(5)(3)- system that eventually led to the appearance of a black precipitate, presumably UO2.
Place, publisher, year, edition, pages
2007. Vol. 46, no 22, 9372-9378 p.
density-functional theory, deactivation processes, aqueous-solution, ion, ligand, hydrolysis, kinetics, coordination, dynamics, binary
IdentifiersURN: urn:nbn:se:kth:diva-17058DOI: 10.1021/ic700817yISI: 000250345400047ScopusID: 2-s2.0-35848960377OAI: oai:DiVA.org:kth-17058DiVA: diva2:335101
QC 201005252010-08-052010-08-05Bibliographically approved