Thermodynamics of Dissociation and Metal Complexation of 3-Nitro-1,5-Diphenylformazan

Abstract

Thermodynamic parameters for dissociation of 3-nitro-1,5-diphenylformazan and its complexation by some divalent metal ions were determined in a 50%(v/v) dioxane–water mixture at constant ionic strength (0.1 M KCl) using an automatic potentiometric technique. The changes in the standard Gibbs energy ΔGo and enthalpy ΔHo accompanying the complexation were found to decrease with increasing metal ionic radius and to increase with the electronegativity, the ionization enthalpy, and the enthalpy of hydration. The order of −ΔGo and −ΔHo values were found to be Mn2+ Zn2+, in accordance with the Irving–Williams order. The complexes were stabilized by both enthalpy and entropy changes and the results suggest that the complexation is an enthalpy-driven process. The transition-series contraction energy Er(Mn–Zn) and the ligand field stabilization energy δ H were calculated from the enthalpy changes.