[Ho-5(H2O)(16)(OH)(2)AS(6)W(64)O(220)](25-), a large novel polyoxoanion from trivacant Keggin fragments

The novel polyoxotungstate Na7K18[Ho-5(H2O)(16)(-)(OH)(2)As6W64O220] center dot 56 H2O (1) was synthesized in aqueous solution and characterized by X-ray structure analysis, elemental analysis and IR spectroscopy. The anion in I represents one of the largest polyoxoanions known yet and exhibits an unusual arrangement of six Keggin units. It consists of six trivacant lacunary alpha-beta-(AsW9O33)(9-) Keggin fragments which are connected by a bridging [Ho5W10(H2O)(16)(OH)(2)O-22](29+) unit. The five Ho-III atoms are coordinated by eight oxygen atoms, forming a square-antiprism.