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

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([Mo₂O₄²⁺]_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.