Thermodynamic and kinetic studies on complex-formation reactions of a cobalt(II) tripodal tetraamine complex

Complex-formation constants were determined for the reaction of [Co(Me(6)tren)(H2O)](2+) with different pyridine- and imidazole-based nucleophiles, and the kinetics of these reactions was studied as a function of entering ligand concentration, temperature, and pressure. The steric hindrance associated with the methyl substituents on the trigonal-bipyramidal complex induces a sensitive control over the substitution mechanism from dissociative for the weaker and more bulky entering nucleophiles, to associative for the stronger and less bulky nucleophiles. A changeover in mechanism can be observed as a function of nucleophile concentration, in the case of 2-MeImH as the entering ligand. The mechanistic assignments are based on the observed rate laws, and the activation parameters determined under limiting concentration conditions.