Relative complexing tendencies of O—O, O—N, and O—S donor (secondary) ligands in some lanthanide–EDTA mixed-ligand complexes

Abstract

Metal–ligand association constants of 1:1 binary (ML) and 1:1:1 ternary (MAL) complexes of the type [Formula: see text] (where M = La3+, Ce3+, Pr3+, Nd3+, or Sm3+; A = primary ligand = EDTA; L = secondary ligand = O—O, O—N, O—S donor aliphatic or aromatic ligand) have been determined potentiometrically by the Irving–Rossotti titration technique at ionic strength 0.2 (mol dm−3 NaClO4) and 25 °C. Differences between log KML and log KMAL are negative; this may be chiefly due to electrostatic repulsion between the primary binary complex and the incoming secondary ligand during the formation of the mixed-ligand complexes. The relative complexing tendencies of various secondary ligands have been found to follow the sequence O—O donor (aromatic) > O—N donor > O—O donor (aliphatic) ≥ O—S donor.