Redistribution reactions of some transition-metal chelate complexes. Part II. Kinetics of bidentate ligand exchange between chelate complexes of nickel (II)

Abstract

Redistribution reactions between the chelate complexes NiL₂ and ML₂′(M = Co, Ni, Cu, or Zn; L and L′= bidentate salicylaldiminato- and β-ketoiminato-ligands or two different β-ketoiminato-residues) have been monitored using contact shifts in the ¹H or ¹⁹F n.m.r. spectra. Kinetic data for reactions of half-life greater than a few minutes fit a rate law for a reversible reaction with second-order forward and reverse steps. Rate constants and some activation parameters are presented. Activation energies of 64–120 kJ mol^–1 were observed. The rates of reaction are dependent on the nature of the substituent on the imino-nitrogen atom within any ligand series, α- and γ-substituents on the β-ketoiminato-ligand, aryl ring substituents on the salicylaldiminato-ligand, added pyridine, and the nature of the central metal atom (M = Co, Ni, Cu, or Zn). Reaction probably proceeds through bond interchanges in a preformed oxygen-bridged mixed dimer.