Sc2Se3O9:: Scandium(III) oxoselenate(IV) according to Sc2[SeO3]3 with a hexagonal "lone-pair" channel structure

The colourless title compound scandium(III) oxoselenate(IV) Sc2Se3O9 (drop Sc-2[SeO3](3)) was obtained by reacting a 1:3-molar mixture of Sc2O3 and SeO2 at 830 degreesC for seven days in an evacuated torch-sealed silica vessel as needle-shaped single crystals. It crystallizes hexagonally (space group: P6(3)/m, Z = 2; a = 814.28(5), c = 764.56(4) pm, c/a = 0.939, V-m = 132.19 cm(3)/mol, R-1 = 0.034 for 364 unique reflections) and exhibits Sc3+ cations in distorted trigonal antiprismatic coordination of six oxygen atoms (d(Sc-O) = 202 - 220 pm) all of which belong to six different isolated Psi(1)-tetrahedral [SeO3](2-) anions (d(Se-O) = 166 - 170 pm). In the crystal structure two [ScO6] octahedra share a common face to form a [Sc2O9] dimer (d(Sc...Sc) = 315 pm). The linkage of different dimers occurs only via common vertices with discrete trigonal non-planar [SeO3](2-) pyramids and the free non-bonding electron pairs ("lone pairs") of the Se4+ cations occupy an empty hexagonal channel propagating along the [001] direction.