Stopped-flow study of the reaction of [Cr(H2O)6]3+ with H3AsO4–H2AsO4– and the much faster reaction of [Cr(NH3)5(H2O)]3+: substitution at arsenate(V)

Abstract

Equilibration of [Cr(H₂O)₆]³⁺ with H₃AsO₄–H₂AsO₄^– to yield the 1 : 1 complex [Cr(H₂O)₅(H₂AsO₄)]²⁺ has been studied in aqueous solution (I= 1.0 mol dm^–3, LiClO₄) at different temperatures with the stopped-flow technique. Results are interpreted in terms of a substitution process on the tetrahedral arsenic(V) centre leaving the Cr–O bonds intact. The corresponding reaction of [Cr(NH₃)₅(H₂O)]³⁺ with H₃AsO₄–H₂AsO₄^– is too fast to follow by the same method, being several orders of magnitude faster. This observation, together with the clearly negative values for the activation entropy for the process for [Cr(H₂O)₆]³⁺, suggests an associatively activated mechanism for the substitution on the arsenic(V) centre.