Abstract: A thermodynamic study is reported for the coordination reaction of meso-tetra(m-substituted)phenylporphyrin zinc [ZnT(m-X)PP, X=NO₂, Cl, OCH₃, H, CH₃] with four substituted imidazoles and three new metal imidazolate complexs of unsymmetrical tetradentate Schiff bases in CH₂Cl₂ solvent by spectrophotometer technique. The equilibrium constant K was determined by Rose Drago's method. The effect of temperature, axial ligands, the substitutents of porphyrin on the stability of complex were studied. It was found that Hammett linear and isoequilibrium relationships exist in the system. The values β and ρ_∞ obtained by different approaches in the system are in agreement with each other.

Key words: meso Tetra( m substituted)phenylporphyrin zinc, Metal Schiff base, Axial coordination, Hammett linear relationship, Isoequilibrium relationship