Thermodynamics of lanthanide(III) complexation with ethylenediamine in dimethyl sulfoxide

The thermodynamics of complex formation of lanthanide(III) ions with the neutral nitrogen-donor ligand ethylenediamine has been studied in the anhydrous aprotic solvent dimethyl sulfoxide at 25 °C and in an ionic medium, 0.1 mol dm–3 NEt4ClO4. The stability constants of the complexes and hence the free-energy changes have been determined by potentiometry and the heats of formation by direct calorimetric measurements. All the complexes are formed in exothermic reactions whereas the entropy terms are unfavourable. Fourier-transform infrared spectroscopic measurements have also been carried out in order to obtain some information on the interaction between the ligand and the metal ions. The results are discussed in terms of the charge density of the metal ions and of solvation changes occurring along the series.