Kinetics of the 1 : 1 equilibration of thiocyanate with the molybdenum(V) aquo-dimer

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Abstract
Equilibration kinetic studies on the complexing of thiocyanate with the molybdenum(V) aquo-dimer [Mo2O4]2+, (I), have been carried out using the temperature-jump technique. With excess thiocyanate the reaction exhibits more than one relaxation effect, and the mechanism cannot be expressed in terms of a single process. With [Mo2O4]2+ in excess (ca. 5 : 1) only one relaxation time corresponding to the formation of the 1 : 1 complex [Mo2O4(NCS)]+ is observed, and this is independent of hydrogen-ion concentration in the range 0·5–2·0M, I= 2·0M, (LiClO4). Relaxation times, τ, can be expressed in terms of equilibrium concentrations of reactant as in (i), where rate constants K1 and k–1 for the formation and aquation of the 1 : 1 complex are, at 25 °C, (2·9 τ–1=K1([Mo2O4 2+]e+[NCS–]e)+K–1(i)± 0·1)× 104 l mol–1 s–1 and 120 ± 10 s–1 respectively. Activation parameters from data at 15–35 °C are ΔH1 ‡= 11·3 ± 0·9 kcal mol–1, ΔS1 ‡=–0·3 ± 3·1 cal K–1 mol–1, ΔH–1 ‡= 13·7 ± 2·5 kcal mol–1, and ΔS–1 ‡=–3·0 ± 8·0 cal K–1 mol–1. The kinetic value of the equilibrium constant (k1/k–1) at 25 °C, I= 2·0M(LiClO4), is 240 l mol–1, which is in good agreement with that obtained spectrophotometrically.

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