5-(2,3-Dimethyl-1-phenylpyrazol-5-one azo)-2-thioxo-4-thiazolidinone (HL) was synthesized and characterized using different spectroscopic techniques. The molecular structures of the ligand are optimized theoretically and the quantum chemical parameters are calculated. The proton-ligand dissociation constants of HLand its metal stability constants with Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺ have been determined potentiometrically. The potentiometric studies were carried out in 0.1 M KCl and 10 % (by volume) DMF-water mixture. At constant temperature the stability constants of the formed complexes increases in the order Cu²⁺> Ni²⁺>Co²⁺> Mn²⁺.The effect of temperature was studied at (298, 308 and 318 K) and the corresponding thermodynamic parameters (DG, DH andDS) 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.