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(C9H4N2O4)(C9H5N2O4)(H2O)3]·2H2O}n, one of the 1H-benzimidazole-5,6-dicarboxyl­ate (Hbdc) ligands is protonated at the imidazole group (H2bdc). The EuIII ion is eight-coordinated by two O atoms from two Hbdc ligands, three O atoms from two H2bdc ligands and three water mol­ecules, showing a distorted square-anti­prismatic geometry. The EuIII ions are bridged by the carboxyl­ate groups of the Hbdc and H2bdc ligands, forming a chain along [110], with an Eu⋯Eu separation of 5.4594 (3) A. These chains are further connected by inter­molecular O—H⋯O, N—H⋯O and N—H⋯N hydrogen bonds, as well as π–π inter­actions 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) A], into a three-dimensional supra­molecular network.