The Effect of Various Inner Coordination Sphere Ligands on the Dissociation Rate of Nickel Malonate Complexes

Abstract

The effects of a number of strongly bonding ligands on the dissociation rates of malonate bidentate nickel complexes have been examined in aqueous nickel malonate solutions using the pressure‐step and shock wave relaxation technique with conductivity readout. The opening of the malonate chelate appears to be the rate controlling step in the dissociation of the malonate. The dissociation rate constant has been compared with the observed shift in the optical absorption peaks for the d – d orbital transitions in the various mixed nickel complexes. On the basis of the comparison the various ligands other than malonate and water in the inner coordination sphere of nickel have been divided into three classes:1. ligands that labilize the malonate chelate with the logarithm of the rate constant increasing as a linear function of the spectral shift to shorter wavelengths (e.g., pyridine, 2‐aminoethyl pyridine, ethylenediamine, 3,3′‐iminopropylamine).2. ligands which produce only a small effect on the dissociation rate of the malonate complex although producing a substantial spectral shift to shorter wavelengths (e.g., α,α′‐bipyridine, phenanthroline, pyridine 2‐aldoxime).3. ligands which labilize the malonate complex but do not produce any appreciable spectral shift (e.g., acetylacetone).These effects are discussed in terms of charge on the ligand, crystal field stabilization energies and σ and π orbital character of the bonding.