catena-Poly[[[triaqua-europium(III)]-μ-(1H-benzimidazole-5,6-dicarboxyl-ato-κO:O)-μ-(1H,3H-benzimidazol-3-ium-5,6-dicarboxyl-ato-κO:O,O)] dihydrate].

In the title one-dimensional coordination polymer, {[Eu(C(9)H(4)N(2)O(4))(C(9)H(5)N(2)O(4))(H(2)O)(3)]·2H(2)O}(n), one of the 1H-benzimidazole-5,6-dicarboxyl-ate (Hbdc) ligands is protonated at the imidazole group (H(2)bdc). The Eu(III) ion is eight-coordinated by two O atoms from two Hbdc ligands, three O atoms from two H(2)bdc ligands and three water mol-ecules, showing a distorted square-anti-prismatic geometry. The Eu(III) ions are bridged by the carboxyl-ate groups of the Hbdc and H(2)bdc ligands, forming a chain along [110], with an Eu⋯Eu separation of 5.4594 (3) Å. These chains are further connected by inter-molecular O-H⋯O, N-H⋯O and N-H⋯N hydrogen bonds, as well as π-π inter-actions between the imidazole and benzene rings [centroid-centroid distances = 3.558 (3), 3.906 (2), 3.397 (3), 3.796 (2) and 3.898 (2) Å], into a three-dimensional supra-molecular network.