COORDINATION AND VALENCE OF EUROPIUM IN [EU(ALPHA-2-AS2W17O61)(2)](17-) AND [EU(W5O18)(2)](9-)

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