Ternary complex formation kinetics involving (1,4,7,10-tetraazacyclododecane)nickel(II) and bidentate ligands

The kinetics of reactions of (1,4,7,10-tetraazacyclododecane)nickel(II), NiT2+, with 1,10-phenanthroline, ethylenediamine, and glycine were studied at 25.0 degrees C and mu = 0.10 mol dm(-3) (NaCl), T is a tetradentate ligand. All reactions were first order in NiT2+ and reversible. The reaction with 1,10-phenanthroline was first order in 1,10-phenanthroline with the formation rate constant k(NiL)(phen) = 1.31 x 10(4) dm(3) mol(-1) s(-1) and the dissociation rate constant k(d) = 19.6 s(-1), followed the Eigen-Tamm mechanism. Ring closure was rate determining. The reaction involving ethylenediamine was resolved into the terms with formation rate constants K-NiL(en) = 1.61 x 10(5) dm(3) mol(-1) s(-1), k(NiL)(Hen) = 4.64 x 10(3) dm(3) mol(-1) s(-1), and dissociation rate constants k(NiT(en)) = 4.55 s(-1) and k(H)(NiT(en)) = 9.62 x 10(7) dm(3) mol(-1) s(-1). Ring closure was rate determining in the en pathway but not in the Hen(+) pathway. The reaction involving glycine varies in order as a function of the glycine concentration, being first order in glycine at low glycine concentration and shifting toward a zero-order dependence a high glycine concentration. The mechanism involves a singly O-bonded LNi-O-N+ intermediate in rapid equilibrium with reactants, K-1 = 1.10 x 10(3) dm(3) mol(-1), followed by a slow ring closure, k(2) = 5.29 x 10(2) s(-1), The dissociation rate constant is k(d) = 6.00 s(-1). The important factors affecting the rates and the mechanisms of these reactions are the steric and inductive effects of the coordinated tetradentate ligand, the conformation angle distortion involving the "bite angle" of the two open coordination sites on NiL2+, and the internal conjugate base effect of the attacking bidentate ligand. (C) 1998 Elsevier Science Ltd. All rights reserved.