Octahedral As in M+ arsenates--architecture and seven new members.

Arsenates with arsenic in octahedral coordination are very rare. The present paper provides an overview of all known M(+) arsenates(V) containing octahedrally coordinated arsenic (M(+) = Li, Na, K, Rb, Cs, Ag) and the crystal structures (determined from single-crystal X-ray diffraction data) of the following seven new hydrothermally synthesized members belonging to six different structure types, four of which are novel: LiH(2)As(3)O(9), LiH(3)As(2)O(7), NaHAs(2)O(6)-type KHAs(2)O(6), KH(3)As(4)O(12) and isotypic RbH(3)As(4)O(12), CsAs(3)O(8) and NaH(2)As(3)O(9)-type AgH(2)As(3)O(9). The main building unit of these compounds is usually an As(4)O(14) cluster of two edge-sharing AsO(6) octahedra sharing two apical corners each with two AsO(4) tetrahedra. The different connectivity between these clusters defines the different structure types. The novel CsAs(3)O(8) structure, based on a derivative of the As(4)O(14) cluster, is the most condensed of all these M(+) arsenates, with an O/As ratio of only 2.67 compared with values of 2.75-3.5 for the remaining members. This is achieved through polymerization of the cluster derivatives to infinite chains of edge-sharing AsO(6) octahedra. The ([4])As/([6])As ratio drops to only 0.5. All but two of the protonated title compounds show protonated AsO(6) octahedra. Hydrogen bonds range from very strong to weak. An analysis of bond-length distribution and average bond lengths in AsO(6) octahedra in inorganic compounds leads to an overall mean As-O distance for all known AsO(6) octahedra (with R factors < 0.072) of 1.830 (2) A.