Oxygen exchange between oxo-anions and water in basic media: molybdate(2–) and tungstate(2–)

Abstract

The ¹⁸O-exchange reaction between [MoO₄]^2– and [WO₄]^2– respectively and water has been investigated in alkaline solutions at constant total concentration of ionic equivalents J= 1.0 mol dm^–3(Na[ClO₄]). Over the range 3 × 10^{– 3}⩽[OH^–]⩽ 1.5 × 10^–l mol dm^–3 at 5 °C ([MoO₄]^2–) and 2 × 10^–3⩽[OH^–]⩽7.5 × 10^–3 mol dm^–3 at 1 °C ([WO₄]^2–) the exchange occurs by spontaneous and hydroxide-ion catalyzed paths (X = MO or W) according to the rate equation (i). The rate constants extrapolated at 25 °C are k₀= 0.33 s^–l and k₁= 2.22 dm³R/[XO₄^2–]_T=k₀+k₁[OH^–](i) mol^–1 s^–1 for [MoO₄]^2– and k₀= 0.44 s^–l and k₁= 273 dm³ mol^–l s^–l for [WO₄]^2–. The following activation parameters were obtained: ΔH_o:^‡= 62.8 ± 2.5 kJ mol^–1ΔS₀^‡=–43.5 ± 9.0 J K^–l mol^–1, ΔH₁^‡= 70.3 ± 5.0 kJ mol^–l, and ΔS₁^‡=–2.5 ± 13.0 J K^–l mol^–l([MoO₄]^2–); and ΔH₀^‡= 29.25 ± 3.22 kJ mol^–l, ΔS_o^†=–153.55 ± 11.30 J K^–l mol^–1, ΔH₁^‡= 71.1 ± 8.0 kJ mol^–l. ΔS₁^‡= 40.2 ± 28.9 J K^–l mol^–l([WO₄]^2–). The values for the kinetic solvent deuterium-isotope effects k₀^H/k₀^D= 3.6 and k₁^H/k₁^D= 2.3 (5 °C, [MoO₄]^2–) and k₀^H/k₀^d= 3.0 and k₁^H/k₁^D= 1.7 (1 °C, [WO₄]^2–), indicate proton transfer in the rate-determining step. The mechanisms of the spontaneous paths are described as solvent-assisted oxygen dissociations with association between H₂O and [XO₄]^2– playing an important role, whereas the [OH]^–- dependent paths are tentatively viewed associative reaction between [OH]^– and [XO₄]^2–.