Poly[[aqua-(μ(5)-1H-benzimidazole-5,6-dicarboxyl-ato)(μ(4)-1H-benzimidazole-5,6-dicarboxyl-ato)dibarium] 4.5-hydrate].

The polymeric title compound, {[Ba(2)(C(9)H(4)N(2)O(4))(2)(H(2)O)]·4.5H(2)O}(n), adopts a layer structure parallel to (001) in which adjacent Ba(II) atoms are connected by two benzimidazole-5,6-dicarboxyl-ate dianions, one functioning in a μ(4)-bridging mode and the other in a μ(5)-bridging mode. The Ba atom having water in its coordination environment as well as the Ba atom without water exist in a nine-coordinate polyhedron of O atoms; the geometry is difficult to derive. Lattice water mol-ecules occupy the space between layers and inter-act with the layers through O-H⋯O, O-H⋯N and N-H⋯O hydrogen bonds. ne of the five lattice water molecules is equally disordered around an inversion centre and shows half-occupancy.