Sodium selenite penta­hydrate, Na₂SeO₃·5H₂O

In the crystal structure of Na₂SeO₃·5H₂O [disodium selen­ate(IV) penta­hydrate], two Se, two selenite O atoms and one water O atom are located on a mirror plane, and one water O atom is located on a twofold rotation axis. The coordination of one Na⁺ cation is distorted trigonal bipyramidal, formed by three equatorial H₂O ligands and two axial selenite O atoms. The other Na⁺ cation has an octa­hedral coordination by six water mol­ecules. The two independent SeO₃ groups form almost undistorted trigonal pyramids, with Se—O bond lengths in the range 1.6856 (7)–1.7202 (10) Å and O—Se—O angles in the range 101.98 (3)–103.11 (5)°, and both are μ₂-O:O-bonded to a pair of Na⁺ cations. Hydrogen bonds involving all water molecules and selenite O atoms consolidate the crystal packing. Although anhydrous Na₂SeO₃ and Na₂TeO₃ are isotypic, the title compound is surprisingly not isotypic with Na₂TeO₃·5H₂O. In the tellurite hydrate, all Na⁺ cations have an octa­hedral coordination and the TeO₃ groups are bonded to Na⁺ only via one of their three O atoms.