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)₅OH₂^2- and solvent water at pH 3 and ionic strength 0.35 mol l^-1 are governed by the following first-order kinetic parameters: k_ex = 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 S_N1 lim ) mechanism for the replacement of water in Co(CN)₅OH₂^2- by other ligands . This implies that the purported intermediate Co(CN)₅^2- 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 (I_d).