A novel heterocyclic compound: catena-poly[[[diaquasodium(I)]-di-mu-aqua] hemi(1,5-dihydroxy-4,8,9-trioxa-2,6-diazabicyclo[3.3.1]nona-2,6-diene-3,7-diolate)].

In the title compound, {[Na(H2O)4]2(C(4)H(2)N(2)O(7))}n, the 1,5-dihydroxy-4,8,9-trioxa-2,6-diazabicyclo[3.3.1]nona-2,6-diene-3,7-diolate anion lies across a twofold axis in the space group C2/c; there are two independent Na sites, one on a twofold axis and the other on a centre of inversion. Hydrogen bonds link the {[Na(H2O)4]+}n chains and diolate anions into a three-dimensional framework.