Poly[aqua­(μ(2)-oxalato)(4-oxidopyri­din­ium)erbium(II)]

The title complex, [Er(C(5)H(5)NO)(C(2)O(4))(H(2)O)](n), is a new erbium polymer based on oxalate and 4-oxidopyridinium ligands. The Er(II) center is coordinated by six O atoms from three oxalate ligands, one O atom from a 4-oxidopyridinium ligand and one water mol­ecule, and displays a distorted square-anti­prismatic coordination geometry. The oxalate ligands are both chelating and bridging, and link Er(II) ions, forming Er–oxalate layers in which the attached water and 4-oxidopyridinium units point alternately up and down. A mirror plane passes through the Er atom, one C, the oxide O and two oxalate O atoms. The layers are assembled into a three-dimensional supra­molecular network via inter­molecular hydrogen bonding and π–π stacking inter­actions [centroid–centroid distances of 3.587 (2) Å between parallel pyridinium rings]. Both the water mol­ecule and the 4-oxidopyridinium ligand are disordered over two sites in a 1:1 ratio.