Abstract: The protonation constant of the ligand and stability constants of its complexes with rare earths have been determined by poteatiometric titration at 25 ℃ and ionic strength μ=0.15 mol·L~(-1)(NaCl). The results indicate that rare earth elements can form 1:1 complexes with L-methionine. There is an apparent "tetrad effect" in this system. Shift of the yttrium position to the vicinity of Gd can be explained by the different polarization between the Ln~(3+) and the ligand. The enthalpy ehanges(△H_(101)) of the coordination reaction as represented by the reaction (M+L<=>ML) have been measured by calorimetric titration, where M and I denote rare earths and L-Met respectively. The ΔG_(101) and ΔS_(101) of these reaction have been calculated by using Gibbs' equation. Furthermore, the stability of rare earth complexes with L-Met has been compared with that of Cu~(2+), Zn~(2+), Fe~(2+), Fe~(3+) complexes with L-Met.

Key words: Rare earth, Methionine, Coordination compounds, Thermodynamic parameters