COORDINATION OF MONOCARBOXYLATES IN LANTHANIDE(III) COMPLEXES

The thermodynamics of Ln(III) coordination of acrylate and propiolate (propynoate) have been investigated by potentiometry and calorimetry. The Delta S-101, values obtained have been used to evaluate the extent of outer-sphere complexation. For Ln=La, Delta(101)=61 and 34 J mol(- 1) K-1 for acrylate and propiolate, respectively, indicating 91% inner -sphere complexation for acrylate and 51% for propiolate. Gd(III)-indu ced C-13 relaxation rate enhancements have been applied to elucidate t he geometries of the concerning complexes. Complexes of chloroacetates , of which the thermodynamics have been investigated previously, have been included in this study. The measurements on mono-, di- and trichl oroacetate reveal an increasing extent of outer-sphere complexation wi th Gd(III) across the ligand series in aqueous solution. Monodentate c oordination is an intermediate stage between bidentate and outer-spher e complexation. Coordination behavior analogous to the chloroacetates is observed for acrylate and propiolate. The extent of inner-sphere co ordination determined in this way agrees well with the conclusions dra wn from the formation entropy data. The obtained complex structures ha ve been used to reevaluate previous La-139 NMR data.