Syntheses, characterization and thermal properties of lanthanide complexes with 2-mercaptonicotinic acid

Fourteen new complexes with the general formula of Ln(Hmna)(3)center dot nH(2)O (n=2 for Ln=La-Ho and n=1 for Er-Lu, H(2)mna=2-mercaptonicotinic acid) were synthesized and characterized by elemental analyses, IR spectra and thermogravimetric analyses. In addition, molar specific heat capacities were determined by a microcalorimeter at 298.15 K. The IR spectra of the prepared complexes revealed that carboxyl groups of the ligands coordinated with Ln(III) ions in bidentate chelating mode. Hydrated complexes lost water molecules during heating in one step and then the anhydrous complexes decomposed directly to oxides Ln(2)O(3), CeO2, Pr6O11 and Tb4O7. The values of molar specific heat capacities for fourteen solid complexes were plotted against the atomic numbers of lanthanide, which presented as 'tripartite effect'. It suggested a certain amount of covalent character existed in the bond of Ln(3+) and ligands, according with nephelauxetic effect of 4f electrons of rare earth ions.