Coordination and valence of europium in [Eu(α-2-As2W17O61)2]17- and [Eu(W5O18)2]9-

Europium L-3 edge X-ray absorption fine structure (XAFS) spectroelectrochemistry was used to determine the valence of europium in [Eu(alpha-2-As2W17O61)(2)](17-) and [Eu(W5O18)(2)](9-). Dilute solutions of these anions in aqueous supporting electrolytes were examined at ambient temperature and at extreme potentials. In situ XANES (X-ray absorption near edge structure) data revealed that Eu is trivalent in both [Eu(alpha-2-As2W17O61)(2)](17-) and [Eu(W5O18)(2)](9-) at rest potential. Furthermore, it was not reduced to Eu2+ by constant-potential bulk electrolysis at significantly reducing potentials under the electrochemical conditions used herein. These results stand in obvious contrast to the redox behavior of [EuP5W30O110](12-), in which Eu3+ is reduced to Eu2+ under similar electrochemical conditions. (C) 1998 Elsevier Science S.A.