Crystal structure of the Anderson-type hetero­polyoxometalate; K(2)[H(7)Cr(III)Mo(6)O(24)]·8H(2)O: a redetermination revealing the position of the extra H atom in the polyanion

The title compound contains a symmetric hydrogen bond in which the H atom does not lie on a crystallographic centre of symmetry. The structure of K(2)[H(7)Cr(III)Mo(6)O(24)]·8H(2)O, namely dipotassium hepta­hydrogen hexa­molybdochromate(III) octa­hydrate, previously reported by Lee [Acta Cryst. (2007), E63, i5–i7], has been redetermined in order to locate the position of the seventh H atom in the anion. Six of the H atoms are bonded to the six μ(3)-O atoms and form hydrogen bonds of medium strength either to water mol­ecules or to the terminal O atoms of other polyanions. The seventh H atom forms a very short hydrogen bond between two μ(2)-O atoms on adjacent polyanions. This short bond, together with two normal hydrogen bonds, link the two crystallographically distinct centrosymmetric polyanions into chains along [011], while the length of this bond [2.461 (3) Å] suggests that the H atom lies at its centre, but unusually for such a bond, this point is not a crystallographic centre of symmetry.