The Crystal Structure of β-K4SiW12O40·9H2O Containing an Isomer of the Keggin Ion

Abstract β-K4SiW12O40·9H2O is orthorhombic, with the space group Pnma and a=20.62(1), b=15.57(1), c=12.95(1) Å, Dm=4.88 g cm−3 and Dx=5.04 g cm−3 for Z=4. The crystal contains a discrete β-SiW12O404− polyanion, a geometrical isomer of the α-SiW12O404− with the well known Keggin structure. The idealized Keggin ion has a Td symmetry, but the β-isomer has only a Cs symmetry. Mutual transformation between the α- and β-structures is possible through a 60° rotation of one of the four W3O13 units, which make a cage around a SiO4 tetrahedron in both structures. The other possible geometrical isomers of the Keggin structure are discussed.