Poly[[tetra­aquadi-μ₄-fumarato-μ₂-oxalato-dierbium(III)] tetra­hydrate]

The title compound, {[Er₂(C₄H₂O₄)₂(C₂O₄)(H₂O)₄]·4H₂O}_n, was synthesized by the reaction of erbium nitrate hexa­hydrate with fumaric acid and oxalic acid under hydro­thermal conditions. The Er³⁺ cation (site symmetry ..2) is eight-coordinated by six O atoms from four fumarate anions (site symmetry ..2) and one bidentate oxalate anion (site symmetry 222), and by two water mol­ecules. The complex exhibits a three-dimensional structure consisting of oxalate pillared Er–fumarate layers with channels occupied by coordinating and lattice water mol­ecules. The three-dimensional structure features by O_water—H...O hydrogen bonds involving both the coordinating and lattice water mol­ecules.