STRUCTURE OF COPPER(II) ETHYLENEDIAMINE COMPLEXES IN AQUEOUS AND NEAT ETHYLENEDIAMINE SOLUTIONS AND SOLVENT-EXCHANGE KINETICS OF THE COPPER(II) ION IN ETHYLENEDIAMINE AS STUDIED BY EXAFS AND NMR METHODS

The structures of copper(II) complexes with ethylenediamine (en) in aqueous and neat ethylenediamine solutions have been determined by the EXAFS (extended X-ray absorption fine structure) method. The copper(II) ion in en has an axially elongated octahedral structure with three en molecules. The Cu-N bond lengths in the four equatorial and two axial positions are 204(1) and 239(2) pm, respectively. The dynamic exchange reaction between en molecules in the bulk and en molecules bound to the copper(II) ion has been studied by the nitrogen-14 NMR line-broadening method. The rate constant and activation parameters for the ethylenediamine solvent exchange are as follows: k(ex) = (1.4 +/- 0.3) x 10(7) s-1 at 25-degrees-C, DELTAH(double dagger) = 9.2 +/- 2.5 kJ mol-1, and DELTAS(double dagger) = -77 +/- 10 J mol-1 K-1. The exchange reaction may be activated via the dissociative-interchange mechanism, followed by the formation of an intermediate with two monodentate en molecules and the occurrence of substantially fast axial-equatorial interconversion in the intermediate.