Abstract
The title compound, tetra-sodium pyroarsenite hemihydrate, [Na(4)(As(2)O(5))(H(2)O)(0.5)], represents the first pyroarsenite compound of an alkali metal. The asymmetric unit comprises four Na, two As, five O sites and one H site in a general position, and one water O atom located on a twofold rotation axis in space group C2/c. The (As(2)O(5))(4-) anion is made up from two trigonal-pyramidal {AsO(3)} units sharing a corner. In the crystal structure, all O atoms of the anion are also part of a framework defined by one [NaO(4)] and three [NaO(6)] coordination polyhedra. Almost linear hydrogen-bonding inter-actions of medium strength [O⋯O 2.651 (4) Å; O-H⋯O 171 (6)°] between the water mol-ecule and one of the terminal O atoms of the pyroarsenite anion consolidate the crystal structure. Data mining for structural data on isolated pyroarsenite anions resulted in 30 (As(2)O(5))(4-) groups present in various crystal structures. In these pyroarsenite anions, the mean As-O distance to terminal oxygen atoms is 1.764 (33) Å and to bridging oxygen atoms 1.856 (64) Å. The values of the As-O-As bridging angle are highly variable [range 107.78 (13) to 144.12 (5)°], with that of Na(4)(As(2)O(5))(H(2)O)(0.5) having by far the smallest value of all structures chacterized so far.