Abstract The reaction of a copper(II) or nickel(II) imidazolate complex (M[CBP-PHEN-4-CHO-Im]) 1 1 The IUPAC name for this ligand is 4 chloro-2-(((2-((1H-imidazol-4-ylmetthylene)amino-)phenyl)imino)phenylmethy-1)phenolato(2-)). with zinc(II)tetraphenylporphyrin (TPP) in toluene results in the formation of an imidazolate bridged heterobinuclear axial adduct. Conversion of the four-coordinated Zn(TPP) to the five-coordinated species is followed in the visible region between 700 and 500 nm. Isosbestic behavior is exhibited at 523, 556, 588, and 638 nm by solutions of Zn(TPP) to which varying amounts of the metal imidazolate complex are added, indicating the existence of an equilibrium between Zn(TPP) and its axial adduct. The products exhibit maxima β and α bands at 566 and 606 nm, respectively, which are red-shifted from 548 and 588 nm for Zn(TPP) and yield ϵ ga/ϵ gb ratios of 0.57 and 0.55 for the Ni(II) and Cu(II) adducts, respectively. The binding of the metal imidazolate complexes is thought to closely resemble that of N-methylimidazole, N-CH 3Im, rather than imidazolate, owing to the close spectral similarities with the adduct of the former and significant differences from the latter. Formation constants were determined using the 548-nm β band of Zn(TPP) in the 293–308 K range by the method of Rose and Drago. At 25° C, K = 152,000 M −1 and 110,000 M -1 for the copper and nickel adducts, respectively. Comparison of these values to that of 54,100 M −1 for N-CH 3Im indicates that the metal-imidazolate complexes are considerably more reactive. Van't Hoff plots for the two series are very similar with enthalpies of −41.9 and −43.3 kJ/mole respectively. The structural core of these complexes is similar to the imidazolate bridged model of cytochrome c oxidase in that they contain a metal imidazolate axially adducted to a metalloporphyrin.