Thermodynamics of complexation of lanthanides by dicarboxylate ligands

Thermodynamic parameters of complexation of lanthanide cations by succinate, glutarate, adipate, and trans-1,4-cyclohexane-dicarboxylate (CHDCA) ligands have been measured with potentiometric and calorimetric techniques. When analogous data for oxalate and malonate complexing are included, the complex stability decreases sharply as the ring size increases from 5 to 7; however, for 7-9-membered rings, there is little difference in stability. The negative entropy contribution associated with expansion in ring size has been attributed to an increasing loss in the configuration entropy in the alkyl chain. The more positive entropy for the Sm-CHDCA complex compared to that of Sm-adipate is attributed to the favorable boat configuration for chelation with CHDCA