Kinetic study of ligand exchange reactions of bis(1,5-cyclooctadiene)nickel(0) with 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, and 4,4′,5,5′ -tetramethyl-2,2′ -bipyridine

Ligand exchange reactions of bis(1,5-cyclooctadiene)nickel(0), Ni(cod)(2), with bpy, DMbpy, and TMbpy (bpy = 2,2'-bipyridine, DMbpy = 4,4'-dimethyl-2,2'-bipyridine, TMbpy = 4,4',5,5'-tetramethyl-2,2'-bipyridine) in THF and DMF have been investigated kinetically. The reaction obeys the pseudo first-order kinetics, R = -d[Ni(cod)(2)]/dt = k(obs)[Ni(cod)(2)], with respect to the concentration of Ni(cod)(2) in both the solutions. In THE the k(obs) value linearly increases with [L] at a low concentration region of the exchanging ligand L; but it levels off at high concentrations of L and approaches a limiting value (e.g., about 1.0 x 10(-4) s(-1) at 25 degrees C). Addition of cod to the THF solution accelerates the exchange reaction. On the other hand, in DMF, the rate of the reaction (R) obeys the second-order rate law, R = k[Ni(cod)(2)][L], over a wide range of [L]. In DME the k value is independent of the concentration of cod added. Based on the kinetic results and NMR analysis, a reaction mechanism involving coordination of bpy, cod, or DMF to Ni(cod)(2) to form a 20 e intermediate complex is proposed. DMbpy and TMbpy have a reactivity comparable to that of bpy in THF. In DMF, their reactivity increases in the order of bpy < DMbpy < TMbpy at 25 degrees C.