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 ΔG^o and enthalpy ΔH^o 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 −ΔG^o and −ΔH^o values were found to be Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > 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 E_r(Mn–Zn) and the ligand field stabilization energy δ H were calculated from the enthalpy changes.