A series of 5-(4′-alkylphenylazo)-3-phenylamino-2-thioxothiazolidin-4-one (HL_n) have been prepared and characterized by elemental analysis, ¹H NMR and IR spectra. X-ray diffraction pattern of ligand (HL₂) was studied. The geometrical structures of these ligands are carried out by HF method with 3-21G basis set. Proton-ligand dissociation constant of ligands (HL_n) and their metal–ligand stability constants of Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺ have been determined potentiometrically in 0.1 M KCl and 30% (v/v) DMSO–water mixture at 298, 308 and 318 K. The stability constants of the formed complexes increase with the order: Mn²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺. The effect of temperature was studied 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. Proton–ligand dissociation constant (pK^H) of the ligands increases according to the following order p-(OCH₃ > H > NO₂) as expected from Hammett_s constant (σ^R).