A series of 5-phenylazo-2-thioxo-4-thiazolidinone derivatives (L1-L5) have been prepared and
characterized by elemental analysis, and IR spectra. Rhodanine and its derivatives are used as an
inhibition of mycobacterium tuberculosis and in medicinal preparations . Azo compounds based on
rhodanine play a central role as chelating agents for a large number of metal ions, as they form a
stable six-membered ring after complexation with the metal ion and can also be used as analytical
reagents. Proton-ligand dissociation constants of (L1-L5) and their metal-ligand stability constants
of their complexes with (MI?, coZ*, Ni2+, CU~+,C~~+, La3+, FIT, and zr4+) metal ions have been
determined potentiornettically in 0.1M KC1 and 40% (vlv) ethanol-water mixture. The influence of
substituents on the dissociation and stability constants was examined on the basis of the electron
repelling property of the substituent. The order of the stability constants of the formed complexes
was found to be ~n'+< co2+< ~i'*< CU~"was studied and the corresponding thermodynamic parameters (AG, MI and AS) were derived and
discussed. The dissociation process is nonspontaneous, endothermic and entropically
unfavourable. The formation of the metal complexes has been found to be spontaneous,
endothermic and entropically favourable.