K(2)[Fe(II) (3)(P(2)O(7))(2)(H(2)O)(2)].

The title compound, dipotassium diaqua-bis-(diphosphato)triferrate(II), K(2)[Fe(II) (3)(P(2)O(7))(2)(H(2)O)(2)], was synthesized under solvothermal conditions. The crystal structure is isotypic with its Co analogue. In the structure, there are two crystallographically distinct Fe positions; one lies on an inversion center, the other on a general position. The first Fe(2+) cation adopts a regular octa-hedral coordination with six O atoms, whereas the other is coordinated by five O atoms and a water mol-ecule. The [FeO(6)] octa-hedron shares its trans-edges with an adjacent [FeO(5)(H(2)O)] octahedron; in turn, the [FeO(5)(H(2)O)] octa-hedron shares skew-edges with a neighbouring [FeO(6)] octa-hedron and an [FeO(5)(H(2)O)] octa-hedron, resulting in a zigzag octa-hedral chain running along [001]. The zigzag chains are linked to each other by the P(2)O(7) diphosphate groups, leading to a corrugated iron diphosphate layer, [Fe(3)(P(2)O(7))(2)(H(2)O)(2)](2-), parallel to (100). The inter-layer space is occupied by K(+) cations, which adopt an eight-coordination to seven O atoms and one water mol-ecule from a neighbouring iron diphosphate layer. Thus, the K(+) ions not only compensate the negative charge of the layer but also link the layers into a network structure.