Crystal structure of the Anderson-type hetero­polyoxometalate; K2[H7CrIIIMo6O24]·8H2O: 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 K2[H7Cr(III)Mo6O24]·8H2O, namely dipotassium hepta-hydrogen hexa-molybdochromate(III) octa-hydrate, 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 μ3-O atoms and form hydrogen bonds of medium strength either to water mol-ecules or to the terminal O atoms of other polyanions. The seventh H atom forms a very short hydrogen bond between two μ2-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.