Kinetics and mechanism of oxidation of oxovanadium(IV) by [Mn(cydta)]^- complex ion: Application of the Marcus relationship in support of an outer-sphere mechanism

Abstract

The rapid scan and the stopped-flow traces of the reaction mixture containing oxovanadium (IV) and [Mn(cydta)]^- indicate the formation of intermediate complex(es). The inverse dependence of the rate on [H⁺] is attributed to the acidic dissociation of aquovanadium (IV) ion. Hence the intermediate complexes are formed between [VO(OH₂)₅]²⁺ and [VO(OH)(OH₂)₄]⁺ with [Mn(cydta)(OH₂)]^- and they decay in the rate limiting steps with rate constants <i>k</i>_H2o and <i>k</i>_OH respectively. The higher ratio of <i>k</i>_OH<i>/ k</i>_H₂O <img src='/image/spc_char/wave.gif' border=0> 30 at 25°C , which previous workers considered to be indicative of an inner-sphere complex, perhaps, reflects the non-adiabaticity of the reaction with aquovanadium (IV) ion. The ratio is consistent with similar observations in several outer-sphere oxidations of V^IV(aq) by metal complexes. The application of the Marcus relationship indicates that the reaction is better grouped with outer-sphere reactions of vanadium (IV) studied with well known outer-sphere oxidants. The values of the rate constants for the formation of the complex, and <i>k</i>_H2O and <i>k</i>_OH the respective rate constants for the decay of the [Mn(cydta)(OH₂) ..VO(OH₂)₅]⁺ and [Mn(cydta)(OH₂) ..VO(OH)(OH₂)₄] are reported.