Abstract
The exchange kinetics of 18-crown-6 complexes with La3+, Ca2+, Pb2+ and Ba2+ ions have been investigated in 70% methanol solution by 1H NMR line-shape analysis. In the case of La3+ and Ca2+, complexes, in the temperature range of –23 to 37 °C, the predominant mechanism for 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 7 °C the ligand exchange proceeds via a dissociative mechanism, at lower temperatures of ⩽–3 °C both the dissociative and bimolecular mechanisms contribute to the ligand exchange. The exchange reaction rates and the activation parameters, Ea, ΔH‡, ΔS‡ and ΔG‡ for the exchange have been determined and the possible parameters controlling the exchange rate and mechanism are discussed.
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