Evidence of Entropy Stabilisation in Ternary Lanthanide Complexes. Lanthanide-EDTA-(O-O)Donor Ternary Complexes

Department of Chemistry, Dr. H. B. Gour Vishwavidyalaya, Sagar-470 003
Manuscript received 13 March 1984, revised 11 April 1985, accepted 31 July 1985

Ternary complexes of the type LN-EDTA-L have been investigated pH-metrically using Irving-Rossotti titration technique for the determination of formation constants. The lanthanides used are trivalent La, Ce, Pr, Nd or Sm and the (O—O) donor secondary ligands (L) are aliphatic or aromatic hydroxy acids, saturated dicarboxylic acids, unsaturated carboxylic acids, phenols or phenolic acids. The stability data have been utilised to demonstrate that these complexes are primarily entropy stabilised.

Mathematical relationships between log \(K_{ML}^M\) or log \(K_{MAL}^M\) vs \(S_M^O\) (standard entropy of the metal ions) have been derived. The plots of stability constant vs \(S_M^O\) are found to be linear, suggesting that these complexes are essentially entropy stabilised. This approach is useful in view of the fact that a direct thermodynamic analysis of the ternary complexes is not possible.