Spectrophotometric study on the stability constants and thermodynamic parameters of Zn2+, Cd2+ and Hg2+ complexes with Imino Thiazolidinone

The heterocyclic ligand (L), 3-(2-hydroxy phenyl)-2-iminothiazolidin-4-one, was synthesized by the cyclocondensation of o-hydoxy phenyl chloroacetamide with potassium thiocyanate. The stoichiometries of the title complexes were first determined by spectrophotometric mole ratio method which gave rise to the M:L ratio of 1:4 in case of Zn(II) and Cd(II) and 1:2 in case of Hg(II) ions respectively. Using these predetermined M:L ratios, complexes of the formulas [Zn-L4], [Cd-L4] and [Hg-L2] were prepared accordingly using precursor of the corresponding metal salts with the title ligand in ethanol medium.  The synthesized compounds were characterized by elemental analysis, FTIR, 1H and 13CNMR, UV-Vis and conductometric measurement. Stability constants (Ks) of these complexes were investigated by spectrophotometric mole ratio method. The FTIR, 1H NMR and 13C NMR data revealed that the studied ligand function as monodentate ligand interacting through phenolic oxygen as donor with Zn(II) and Cd(II) and as bidentate ligand interacting through phenolic oxygen and nitrogen  atom with Hg(II). The synthesized complexes show conductivity values in the range of 122-133 µSmol-1cm2 in DMSO at 298 K which confirms the electrolytic nature of the complexes. The stability constants decreased with increased temperature, confirming that these metal complexes are not stable at higher temperature. Sufficiently large negative values of ΔG of complex confirm the spontaneous formation of the title complexes. Furthermore, it was noted that the spontaneity of the reaction increased with temperature. The stability constant of these complexes follow the sequence Zn(II)>Cd(II)>Hg(II). Therefore, the overall result is complying very well with the Irving-William series of stability constants of metal complexes.   Key words: Stability constants, 3-(2-hydroxyphenyl)-2-iminothiazolidin-4-one, Irving-William series, thermodynamic parameters, mole ratio method.