Poly[[tetra­aquadi-μ(4)-fumarato-μ(2)-oxalato-dierbium(III)] tetra­hydrate]

The title compound, {[Er(2)(C(4)H(2)O(4))(2)(C(2)O(4))(H(2)O)(4)]·4H(2)O}(n), was synthesized by the reaction of erbium nitrate hexa­hydrate with fumaric acid and oxalic acid under hydro­thermal conditions. The Er(3+) 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.