Complexation kinetics of N-alkyl-substituted diamino diamides with nickel(II) in aqueous solution

Equilibrium constants are determined for the protonation and metal complexation of the nickel(II) complexes with 4-methyl-4,7-diazadecanediamide (4-Me-L-2,2,2), 4,7-dimethyl-4,7-diazadecanediamide (4,7-N,N'-Me-2-L-2,2,2), 4-ethyl-4,7-diazadecanediamide (4-Et-L-2,2,2), and 4-methyl-4,8-diazaundecanediamide (4-Me-L-2,3,2), in 0.10 M KCl at 25.0 degreesC. The formation kinetics of these nickel(II) complexes have been studied under the same conditions with use of the stopped-flow technique. The possible pathways for the complexation reaction of nickel(II) with these ligands are discussed. The first metal-nitrogen bond formation is proposed as the rate-determining step for the reactions of nickel(II) with the unprotonated ligands; proton loss is the rate-limiting step in the reactions of nickel(II) with the monoprotonated ligands. Similarly, in dissociation reactions of these nickel(II) complexes, the rate-determining step for the water dissociation pathway is the breakage of the second nickel-nitrogen bond; the rate-determining step for the proton-assisted pathway is the protonation of the released amino group. The important factors determining the reactivity of these complexes are considered. The kinetic results of the formation and dissociation reactions of these complexes are consistent with dissociative mechanism.