The Unique Crystal Structure of the Triclinic Samarium(III) Oxoselenate(IV) Sm 2 [SeO 3 ] 3

Pale yellow, needle‐shaped single crystals of Sm 2 [SeO 3 ] 3 were obtained by heating stoichiometric mixtures of Sm 2 O 3 and SeO 2 (molar ratio: 1:3) along with substantial amounts of CsCl as fluxing agent in evacuated sealed silica tubes at 830 °C for one week. According to X‐ray single‐crystal diffraction data, Sm 2 [SeO 3 ] 3 crystallizes triclinic (space group: $P{\bar 1}$ ) with two formula units per unit cell of the dimensions a = 698.62(7), b = 789.71(8), c = 910.34(9) pm, α = 96.693(5), β = 104.639(5), γ = 115.867(5)°. Its crystal structure contains two crystallographically distinct Sm 3+ cations in eight‐ and ninefold coordination with oxygen atoms arranged as distorted uncapped or capped square antiprisms (d(Sm 3+ −O 2− ) = 232−271 pm). These [(Sm1)O 8 ] and [(Sm2)O 9 ] polyhedra share opposite edges and faces to form zigzag chains along [100] with discrete pyramidal [SeO 3 ] 2− anions bridging units. Further linkage by [SeO 3 ] 2− anions in [010] direction leads to a three‐dimensional network, which exhibits almost rectangular channels along [111]. These tunnels offer width enough to incorporate the free non ‐bonding electron pairs („lone pairs”) at the Se 4+ cations, since all nine different Ψ 1 ‐tetrahedral [SeO 3 ] 2− groups (d(Se 4+ −O 2− ) = 165−173 pm, ∢(O–Se–O) = 94 – 108°) exhibit a pronounced stereochemical „lone‐pair” activity. For not being isotypic with neither triclinic Er 2 [SeO 3 ] 3 (CN(Er 3+ ) = 7 and 8) nor the remainder rare‐earth metal(III) oxoselenates(IV) of the composition M 2 [SeO 3 ] 3 (≡ M 2 Se 3 O 9 ; M = Sc, Y, La, Ce – Lu), Sm 2 [SeO 3 ] 3 claims a unique crystal structure among them.