Azo compound, based on rhodanine, play a central role as chelating agents for a large numbers of metal ions, as they form a stable six-member ring after complexation with a metal ion and can also be used as analytical reagents. The complex-formation equilibria have been reported for several kinds of rhodanine derivatives. Potentiometry,conductivity and spectroscopy measurements on the coordination ability biologically important azo derivatives have shown that their complexes are very stable. The aim of the present work includes: Synthesized of 5-(4'-derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) ligands and characterized by elemental analyses and various physico-chemical techniques, Preparation of new complexes of VO(IV), Ru(III) and a novel Pd(II) with HLn,Knowledgement the reactivity of HLn towards Pd(II) and its ability in electron donors mixed ligand complex formation,Studying the ESR spectrum of VO(IV) complexes to compare the experimental compounds with the expected and prove them, Studying the thermal decomposition behaviour of the complexes, Determination of proton–ligand dissociation constants of rhodanine azodyes and their substituted derivatives potentiometri