Kinetics of CO Addition to Ni(cyclam)+ in Aqueous Solution

The addition of CO to Ni(cyclam) + in homogeneous aqueous solution has been studied by pulse radiolysis and laser flash photolysis. The reaction was found to proceed by a sequential, two-step mechanism. The CO addition rate constant was found to be (2.0 ± 0.2) × 10 9 M - 1 s - 1 at ambient temperature. Addition is followed by a slower, first-order reaction with a rate constant of 1.8 ± 0.2 s - 1 at ambient temperature. A variable temperature study of the latter reaction yielded Δ H ⧧ = 12.7 ± 0.1 kcal mol - 1 and Δ S ⧧ = −14.4 ± 0.1 cal K - 1 mol - 1 . The mechanism of the reaction sequence and the possibility that the slow, first-order process represents an isomerization of the macrocyclic ligand are discussed.