Termodynamic Aspects of the Ni(III)/Ni(II) Redox Change in Poly-Aza-Macrocyclic Complexes.

The reaction entropies of the Ni(III)/Ni(II) redox couple for some polyaza macrocyclic complexes in aqueous solution have been determined by measuring E1/2(Ni(III)/Ni(II) values through voltammetry experiments at varying temperatures using a nonisothermal cell. reaction entropies values are in any case positive, and their magnitudes decrease with the increasing number of nitrogen atoms of the coordinating system (from four to six): this is ascribed to the release of water molecules from the coordination sphere and/or the hydration sphere during the Ni"'/Ni" redox change. The presence of negative charges on the coordinating system (deprotonated amido or carboxylate groups) makes the reaction entropies ,(Ni"'/Ni1') values substantially less positive. Redox equilibria of the type M"'L, + M"L2 in equilibrium with M"L1 + M"'L2 have been considered and the corresponding values of the enthalpy term evaluated for chosen pairs of ligating systems L1, and L2. Inspection of the above values indicates that the enthalpy contribution (which includes bond energy terms) favors the attainment of trivalent nickel along the sequence N(amine) > N(deprotonated amido groups) = -COOand that a square arrangement of four nitrogen atoms (cyclam) is the very most favorable coordinative situation (from the enthalpic point of view).