The Kinetics of Water Exchange on Aqueous Aquapentacyanocobaltate(III)

Abstract

The temperature and pressure dependences of the rate of exchange of oxygen-18 between labelled Co(CN)5OH22- and solvent water at pH 3 and ionic strength 0.35 mol l-1 are governed by the following first-order kinetic parameters: kex = 5.8x10-4 s-1 at 298 K; ΔH‡ = 90.2 kJ mol-1; ΔS‡ = -4 J K-1 mol-1; and ΔV‡ = + 7 cm3 mol-1 (mean value, 0-400 MPa ). Although these results are indicative of a strongly dissociative mode of activation, the rate of water exchange is about twice that predicted by previous workers on the basis of a limiting dissociative (D or SN1 lim ) mechanism for the replacement of water in Co(CN)5OH22- by other ligands . This implies that the purported intermediate Co(CN)52- cannot be long-lived on the time scale of relaxation of the second coordination sphere, and supports the contention of Burnett and coworkers that the mechanism of this textbook example of a series of dissociatively activated ligand substitution reactions is properly classified as dissociative interchange (Id).