Equilibrium studies on Schiff base complexes of copper(II) and nickel(II): Ligand substitution and formation of mixed ligand complexes

Abstract

Vis spectrophotometry has been used to study various ligand substitution equilibria (1) and (2) involving four-coordinate copper(II) and nickel(II) his chelate complexes in methanol, propan-2-ol and toluene. MA2 + HB ⇋ MAB+HA, K1 (1) MAB + HB ⇋ MB2 + HA, K2 (2) The Schiff base ligands, HA and HB, which are monobasic and bidentate, represent salicylaldimine type N,O-ligands (≙ HSA=NR) (1) or pyrrole-2-aldimine type N,N-ligands (≙ HPA=NR) (2) with different branching at the α- or (3-carbon of the organic group R. For both types of ligand the relative thermodynamic stability of their copper and nickel complexes is governed mainly by the steric demands of R, which determine the degree of tetrahedral distortion. The order of stability as given by β = K1 K2 is: t-Bu < neo-Pent < i-Pr < i-Bu < Et < n-Pr. The K1/K2 ratio is strongly solvent dependent in the sense that the mixed ligand species MAB is stabilised in toluene relative to methanol. Such a solvent effect is not observed for β. The MAB complexes could not be isolated. The vis spectrum of the mixed ligand species Ni(SA=NiPr, SA=NEt) was calculated by computer fitting of the experimental data.