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

Equilibrium constants are determined for the protonation and metal complexa tion 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-e thyl-4,7-diazadecanediamide (4-Et-L-2,2,2), and 4-methyl-4,8-diazaundecaned iamide (4-Me-L-2,3,2), in 0.10 M KCl at 25.0 degreesC. The formation kineti cs of these nickel(II) complexes have been studied under the same condition s with use of the stopped-flow technique. The possible pathways for the com plexation reaction of nickel(II) with these ligands are discussed. The firs t metal-nitrogen bond formation is proposed as the rate-determining step fo r the reactions of nickel(II) with the unprotonated ligands; proton loss is the rate-limiting step in the reactions of nickel(II) with the monoprotona ted ligands. Similarly, in dissociation reactions of these nickel(II) compl exes, the rate-determining step for the water dissociation pathway is the b reakage of the second nickel-nitrogen bond; the rate-determining step for t he proton-assisted pathway is the protonation of the released amino group. The important factors determining the reactivity of these complexes are con sidered. The kinetic results of the formation and dissociation reactions of these complexes are consistent with dissociative mechanism.