Sc2[Se2O5]3: The First Rare-Earth Metal Oxoselenate(IV) with Exclusively [Se2O5]2- Anions

The scandium oxodiselenate(IV) Sc-2[Se2O5](3) was synthesized via solid-state reactions between scandium sesquioxide (Sc2O3) and selenium dioxide (SeO2) with thallium(I) chloride (TlCl) as fluxing agent in molar ratios of 1:4:2. Evacuated fused silica ampoules were used as reactions vessels for annealing the mixtures for five days at 800 degrees C. The new scandium compound crystallizes in the triclinic space group P (1) over bar with the lattice parameters a = 663.71(5) pm, b = 1024.32(7) pm, c = 1057.49(8) pm, alpha = 81.034(2)degrees, beta = 87.468(2)degrees, gamma = 89.237(2)degrees and Z = 2. There are two distinct Sc3+ positions, which show six-fold coordination by oxygen atoms as [ScO6](9-) octahedra (d(Sc-O) = 205-212 pm). Three different [Se2O5](2-) anions provide these oxygen atoms with their terminal ligands (O-t). Each of the six selenium(IV) central atoms exhibit a stereochemically active lone pair of electrons, so that all [Se2O5](2-) anions consist of two psi(1)-tetrahedral [SeO3](2-) subunits (d(Se-O-t) = 164-167 pm, d(Se-O-b) = 176-185 pm, not less than(O-Se-O) = 93-104 degrees) sharing one bridging oxygen atom (O-b) with not less than(Se-O-b-Se) = 121-128 degrees. The vibrational modes of the complex anionic [Se2O5](2-) entities were characterized via single-crystal Raman spectroscopy.