Kinetics of the ligand exchange of bipyridine in tetradentate nickel complexes

An investigation into the ligand exchange reaction of bipyridine with Ni(EDDA) and Ni(trien) has led to proposed mechanisms based on steric and electronic effects. Kinetic studies show that the rate order dependence of bipyridine is different for the two reactions implying a slightly different mechanism for the bulkier trien. Both mechanisms involve unwrapping of the tetradentate ligand as the rate-determining step. This unwrapping is faster in acidic solution, because protonation of the amine groups inhibits recoordination of the ligand to the nickel. The rates for bipyridine exchange were significantly faster than for phenanthroline, suggesting an electronic ligand effect.