Spectroscopic, potentiometric and thermodynamic studies of azo rhodanines and their metal complexes

The proton-ligand dissociation constant of 5-(4′-arylazo)-2-thioxothiazolidin-4-one (HLn) and metal-ligand stability constants of their complexes with metal ions (Mn2 +, Co2 +, Ni2 +, Cu2 + and Zn2 +) have been determined potentiometrically in 0.1 M KCl and 50% (by volume) methanol-water mixture. The molecular and electronic structures of the investigated compounds (HLn) were also studied using quantum chemical calculations. For the same ligand at constant temperature, the stability constants of the formed complexes increase in the order Cu2 + > Zn2 + > Ni2 + > Co2 + > Mn2 +. The effect of temperature was studied at (303, 313 and 323) K and the corresponding thermodynamic parameters (ΔG, ΔH and ΔS) were derived and discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavorable. The formation of the metal complexes has been found to be spontaneous, endothermic and entropically favorable. The pKH value of the ligands increases in the order HL1 > HL2 > HL3.