Sm2As4O9: Ein ungewöhnliches Samarium(III)‐Oxoarsenat(III) gemäß Sm4[As2O5]2[As4O8]

Sm₂As₄O₉: An Unusual Samarium(III) Oxoarsenate(III) According to Sm₄As₂O₅]₂As₄O₈] Pale yellow single crystals of the new samarium(III) oxoarsenate(III) with the composition Sm₄As₈O₁₈ were obtained by a typical solid‐state reaction between Sm₂O₃ and As₂O₃ using CsCl and SmCl₃ as fluxing agents. The compound crystallizes in the triclinic crystal system with the space group (No. 2, Z = 2; a = 681.12(5), b = 757.59(6), c = 953.97(8) pm, α = 96.623(7), β = 103.751(7), γ = 104.400(7)°). The crystal structure of samarium(III) oxoarsenate(III) with the formula type Sm₄As₂O₅]₂As₄O₈] (≡ 2 × Sm₂As₄O₉) contains two crystallographically different Sm³⁺ cations, where (Sm1)³⁺ is coordinated by eight, but (Sm2)³⁺ by nine oxygen atoms. Two different discrete oxoarsenate(III) anions are present in the crystal structure, namely As₂O₅]^4? and As₄O₈]^4?. The As₂O₅]^4? anion is built up of two Ψ¹‐tetrahedra AsO₃]^3? with a common corner, whereas the As₄O₈]^4? anion consists of four Ψ¹‐tetrahedra with ring‐shaped vertex‐connected AsO₃]^3? pyramids. Thus at all four crystallographically different As³⁺ cations stereochemically active non‐binding electron pairs (“lone pairs”) are observed. These “lone pairs” direct towards the center of empty channels running parallel to 010] in the overall structure, where these “empty channels” being formed by the linkage of layers with the ecliptically conformed As₂O₅]^4? anions and the stair‐like shaped As₄O₈]^4? rings via common oxygen atoms (O1 – O6, O8 and O9). The oxygen‐atom type O7, however, belongs only to the cyclo‐As₄O₈]^4? unit as one of the two different corner‐sharing oxygen atoms.