Kinetics of the Ligand Substitution Reactions of Tetrahedral Complexes [Co(NN)(hmpa)2]2+(NN=2,2′-Bipyridine, 4,4′-Dimethyl-2,2′-bipyridine, or 1,10-Phenanthroline) in Hexamethylphosphoric Triamide, Nitrobenzene, Acetone, 1,2-Dichloroethane, and Nitromethane

The substitution reaction rates of hexamethylphosphoric triamide (HMPA) for NN (NN=2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmbpy), or 1,10-phenanthroline (phen)) on tetrahedral complexes [Co(NN)(hmpa)2]2+ were determined spectrophotometrically in pure HMPA or in nitrobenzene, acetone, 1,2-dichloroethane, and nitromethane at various concentrations of HMPA, for the purpose of clarifying the reaction mechanism and the effect of the bulkiness of HMPA. The ligand substitution reaction on bpy and dmbpy complexes proceeds simply via a dissociative mechanism in nitrobenzene. In inert solvents other than nitrobenzene, it proceeds in parallel via two simultaneous paths of both dissociative and associative mechanisms. In the case of the phen complex, the reaction proceeds only by a dissociative mechanism in pure HMPA and in all the inert solvents used. The activation enthalpies and the activation entropies of the dissociative mechanism of bpy and dmbpy complexes were (80–90) kJ mol−1 and approximately ...