KINETICS OF TERNARY COMPLEX-FORMATION WITH THE (NITRILOTRIACETATO)COPPER(II) COMPLEX

Ligand substitution reactions of the (nitrilotriacetato)copper(II) complex (CuNTA-) with simple mono- and bidentate ligands (L) have been investigated by making use of the temperature-jump relaxation technique. At 25-degrees-C, the forward rate constants for reaction CuNTA- + L = CuNTA(L)- are as follows: L = NH3, 1.5 X 10(8) M-1 s-1; L = CH3NH2, 3.4 x 10(7) M-1 S-1; L = glycinate, 1.1 x 10(8) M-1 s-1; L = alpha-alaninate, 4.9 x 10(7) M-1 s-1; L = beta-alaninate, 3.8 x 10(7) M-1 s-1; L = 2,2'-bipyridine, 1.2 x 10(6) M-1 s-1; L = 1,10-phenanthroline, 2.2 x 10(5) M-1 s-1. The rate constants for the reactions of the zwitterion form of glycine and a-alanine, CuNTA- + HL+/- = CuNTA(L)2- + H+, are 1.6 x 10(3) M-1 s-1 and 1.8 X 10(3) M-1 s-1, respectively. It was concluded that the reactions of the aliphatic ligands proceed through a dissociative interchange mechanism in which the axial coordination of the entering ligand is the rate-determining step. The unusually slow reactions with the aromatic ligands have been interpreted in terms of steric interference between the coordinated and entering ligands.