Formation Thermodynamics of Binary and Ternary Lanthanide(III) Complexes with 1,10-Phenanthroline and Chloride in N , N -Dimethylformamide

Abstract

Formation thermodynamics of binary and ternary lanthanide(III) (Ln = La, Ce, Nd, Eu, Gd, Dy, Tm, Lu) complexes with 1,10-phenanthroline (phen) and the chloride ion have been studied by titration calorimetry and spectrophotometry in N,N-dimethyl-formamide (DMF) containing 0.2 mol-dm−3 (C2H5)4NClO4 as a constant ionic medium at 25°C. In the binary system with 1,10-phenanthroline, the Ln(phen)3+ complex is formed for all the lanthanide(III) ions examined. The reaction enthalpy and entropy values for the formation of Ln(phen)3+ decrease in the order La > Ce > Nd, then increase in the order Nd Tm > Lu. The variation is explained in terms of the coordination structure of Ln(phen)3+ that changes from eight to seven coordination with decreasing ionic radius of the metal ion. In the ternary Ln3+-Cl−-phen system, the formation of LnCl(phen)2+, LnCl2(phen)+, and LnCl3(phen) was established for cerium(III), neodymium(III), and thulium(III), and their formation constants, enthalpies, and entropies were obtained. The enthalpy and entropy values are also discussed from the structural point of view.