Mechanistic elucidation of the substitution behavior of alkyl cobaloximes in water and methanol as solvents

The ligand substitution behavior of trans-[Co(Hdmg)2(R)S] (R = CH3, PhCH2; Hdmg = dimethylglyoximate; S = H2O and/or MeOH) was studied for imidazole, pyrazole, 1,2,4-triazole, N-acetylimidazole, 5-chloro-1-methylimidazole, NO2-, Ph3P, Ph3As, and Ph3Sb as entering ligands. In all cases, except for Ph3As and Ph3Sb, the observed kinetics shows a linear dependence on the entering nucleophile concentration with no evidence for a back reaction. In the case of Ph3As and Ph3Sb as entering nucleophiles, kinetic evidence for a reverse solvolysis reaction is at hand. Activation parameters (DeltaH++, DeltaS++, and DeltaV++) were determined from the temperature and pressure dependence of all studied reactions and support the operation of a dissociatively activated substitution mechanism. The rate and activation parameters show that there is an increase in the dissociative character from a dissociative interchange to a limiting dissociative mechanism that depends on the nature of R and the entering nucleophile. The crystal structure of trans-[Co(en)2(Me)H2O]2+ was determined by X-ray analysis. The Co-O and Co-C bond lengths were found to be 2.153(6) and 1.995(10) angstroms, respectively. The kinetic and structural data are discussed in reference to a series of earlier studied systems for which data are reported in the literature.