NMR-STUDY OF THE LIGAND INTERCHANGE OF 18-CROWN-6 COMPLEXES WITH LA3-PERCENT METHANOL SOLUTION(, CA2+, PH(2+) AND BA2+ IONS IN 70)

The exchange kinetics of 18-crown-6 complexes with La3+, Ca2+, Pb2+ an d Ba2+ ions have been investigated in 70% methanol solution by H-1 NMR line-shape analysis. In the case of La3+ and Ca2+, complexes, in the temperature range of -23 to 37 degrees C, the predominant mechanism fo r the exchange of the ligand between the free and complexed sites is a bimolecular pathway. However, in the case of Pb2+ and Ba2+ complexes, while at temperatures greater than or equal to 7 degrees C the ligand exchange proceeds via a dissociative mechanism, at lower temperatures of less than or equal to-3 degrees C both the dissociative and bimole cular mechanisms contribute to the ligand exchange. The exchange react ion rates and the activation parameters E(a), Delta H-double dagger, D elta S-double dagger and Delta G(double dagger) for the exchange have been determined and the possible parameters controlling the exchange r ate and mechanism are discussed.