Kinetics and Mechanism of Axial Ligand Substitution of Alkyl Cobaloximes by Substituted Pyridines in Different Solvents

Ligand‐substitution reactions of trans ‐RCo(Hdmg) 2 S (where R = PhCH 2 or CF 3 CH 2 , Hdmg = dimethylglyoximate, and S = H 2 O or MeOH) were studied for the nucleophiles 4‐aminopyridine (4‐NH 2 Py), pyridine (Py) and 4‐cyanopyridine (4‐CNPy). From the pressure and temperature dependence of the substitution of methanol by 4‐NH 2 Py, Py, and 4‐CNPy, the activation parameters Δ H ≠ , Δ S ≠ , and Δ V ≠ were estimated for the cases where R = PhCH 2 and CF 3 CH 2 . The activation parameters for the substitution of H 2 O by Py and 4‐CNPy in the case where R = CF 3 CH 2 were also found. The kinetic data and activation parameters show that the dissociative character of the reaction decreases on changing the R group from PhCH 2 to CF 3 CH 2 and the solvent from MeOH to H 2 O. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)