ABSTRACT:The geometry of metal ions (La 3ϩ , Ce 3ϩ , UO 2 2ϩ , and Th 4ϩ ) complexes with 5-azorhodanine derivatives was optimized at the level of molecular mechanics. Two stoichiometric ratios of metal to ligand (i.e., 1:1 and 1:2) were investigated. Tetracoordinate and hexacoordinate of each stoichiometric ratio have been studied. Effect of substitution in the ligand on the geometry of the complexes was discussed in the light of electron donating-accepting properties of these substituents. The influence of the nuclear effective charge of the central metal ions on the metal-ligand (M-L) bonding was discussed and the effect of the number of ligands on the M-L bond length was also discussed and correlated to the experimental results. The total energies of the different metal complexes were computed using the extended Huckel method. The effect of substituents in ligand, metal type, and stoichiometry of the complexes on the complex total energies were discussed. Stability constant of (La , and Th 4ϩ ) metal ions with 5-azorhodanine derivaties have been determined potentiometrically in 0.1 M KCl and 50% (v/v) ethanol-water mixture. The order of the stability constants of the formed complexes was found to be La 3ϩ Ͻ Ce 3ϩ Ͻ UO 2 2ϩ Ͻ Th 4ϩ . The influence of substituents on the stability of the complexes was examined on the basis of electron-repelling property of the substituent. The effect of temperature on the stability of the complexes formed was studied and the corresponding thermodynamic parameters (⌬G, ⌬H, and ⌬S) were derived and discussed. The stoichiometries of these complexes were determined conductometrically and indicated

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