Kinetic study of the reaction of meso-tetrakis ( p-trimethylammoniumphenyl)porphinatodiaquacobalt(III) with thiocyanate and iodide ions in aqueous solution

Abstract

The reaction of meso-tetrakis( p-trimethylammoniumphenyl)porphinatodiaquacobalt(III) with NCS − and I − ions has been studied at 15, 25 and 35°C in 0.1 mol dm −3 [H +] (HNO 3) and μ = 1.0 mol dm −3 (NaNO 3). The values of the acidity constants, K a1 and K a2, for the diaqua complex at 25°C are 3.3(5) x 10 −7 and 2.0(2) x 10 −11 mol dm −3, respectively. The reactions are first order in ligand concentration up to 0.9 mol dm −3. The values of the second-order rate constants at 25°C are 89.0(5) and 76.0(4) dm 3 mol −1 s −1 for NCS − and I −, respectively. The values of Δ S ‡ for the different incoming ligands are positive or nearly positive, which is indicative of dissociative activation. The value of the activation volume [Δ V ‡ = 18.8(8) cm 3 mol −1] indeed points to a dissociative mechanism. The porphine ligand greatly labilizes the cobalt(III). There is about a 10 8-fold increase in the rate constant for substitution compared with that of [Co(NH 3) 5(H 2O)] 3+. It is proposed that the electron-rich porphine not only labilizes the M III ion (M = Co, Rh and Cr) but also promotes a more dissociative activation mode relative to that of the corresponding [M(NH 3) 5(H 2O)] 3+ complexes. The extent of labilization depends on the electron-donating ability of the porphine as well as the nature of the central metal ion. It is also shown that the overall charge of the complex, whether positive or negative (as determined by the formal charge of the porphine), plays a minor role in the mechanism of the substitution reaction.