A series of 5-(4′-alkylphenylazo)-3-phenylamino-2-thioxothiazolidin-4-one (HL n) 
have been prepared and characterized by elemental analysis, 1 H NMR and IR spectra. X-
ray diffraction pattern of ligand (HL 2) 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 Mn 2+, Co 2+, Ni 2+