Synthesis, acid hydrolysis and formation of the [(H3N)5CoOMoO3]+ ion in aqueous solution

Abstract

The complex [(H₃N)₅CoOMoO₃]ClO₄ has been synthesised and characterised by elemental analysis, UV/VIS and infrared spectroscopy. The kinetics of its acid hydrolysis has been studied using the stopped-flow technique over the ranges 0.01 ⩽[H⁺]⩽ 0.45 mol dm^–3, 24.9 ⩽θ⩽ 35.6 °C, 0.04 ⩽I⩽ 1.0 mol dm^–3(NaClO₄). The rate of reaction is inversely dependent on [H⁺] due to the expansion of the co-ordination of the molybdenum(VI) from four to six on protonation, along with resonance stabilisation. The rate constant (k₁) for the hydrolysis is 5.09 ± 0.02 s^–1 at 24.9 °C for which ΔH^‡= 82 ± 1 kJ mol^–1 and ΔS^‡= 43 ± 5 J K^–1 mol^–1. The kinetics of formation of the complex from pentaammineaquacobalt(III) and molybdate(VI) ions has been studied using the stopped-flow method over the range pH 7.13–8.46 and at 25.0–35.9 °C, I= 1.0 mol dm^–3. Over this pH range both [Co(NH₃)₅(OH₂)]³⁺ and [Co(NH₃)₅(OH)]²⁺ ion react with HMoO₄^– to give [(H₃N)₅CoOMoO₃]⁺, and the rate constants are (2.03 ± 0.05)× 10⁶ and (2.73 ± 0.05)× 10⁵ dm³ mol^–1 s^–1, respectively at 25.0 °C. Both acid hydrolysis and complexation are very rapid, suggesting a mechanism involving cleavage of the Mo^VI–O not the Co^III–O bond. Details of both mechanisms are discussed.