Crystal structure of potassium sodium hepta­hydrogen hexa­molybdocobaltate(III) octa­hydrate: an extra-protonated B-series Anderson-type heteropolyoxidometalate

The title compound, KNa[Co(III)(OH)(7){Mo(6)O(17)}]·8H(2)O, was obtained by the ion-exchange technique from K(3)[Co(μ(3)-OH)(6)Mo(6)O(18)]·7H(2)O. Six μ(3)-O atoms and one O atom of the bridging μ(2)-O atom are protonated. This novel polyanion protonated by an extra H atom is an unexpected polyanion species among the B-series Anderson-type polyoxidometalates (POMs), [X(n) (+)(μ(3)-OH)(6)Mo(6)O(18)]((6–n)–)] (X = heteroatom). The extra H atom (seventh H atom) in the polyanion does not lie on a crystallographic centre of symmetry, but is located at the mid-point between two μ(2)-O atoms of adjacent polyanions, and forms a very short hydrogen bond [2.430 (5) Å]. The present structure is considered as particularly significant in understanding noncentrosymmetric strong hydrogen bonding.