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

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/[XO42–]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: ΔHo:‡= 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, ΔSo†=–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 k0H/k0D= 3.6 and k1H/k1D= 2.3 (5 °C, [MoO4]2–) and k0H/k0d= 3.0 and k1H/k1D= 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–.