The Triclinic Lanthanoid(III) Halide Oxidoarsenates(III) Sm 3 Cl 2 [As 2 O 5 ][AsO 3 ] and Tm 3 Br 2 [As 2 O 5 ][AsO 3 ]

Pale yellow single crystals of the composition Ln 3 X 2 [As 2 O 5 ][AsO 3 ] ( Ln = Tm for X = Br and Ln = Sm for X = Cl) were obtained via solid‐state reactions in the systems Ln 2 O 3 /As 2 O 3 from sealed silica ampoules using different halides as fluxing agents. Sm 3 Cl 2 [As 2 O 5 ][AsO 3 ] and Tm 3 Br 2 [As 2 O 5 ][AsO 3 ] crystallize isotypically in the triclinic space group P 1 with Z = 2 and cell parameters of a = 543.51(4) pm, b = 837.24(6) pm, c = 1113.45(8) pm, α = 90.084(2)°, β = 94.532(2)°, γ = 90.487(2)° for the samarium and a = 534.96(4) pm, b = 869.26(6) pm, c = 1081.84(8) pm, α = 90.723(2)°, β = 94.792(2)° γ = 90.119(2)° for the thulium compound. The isotypic crystal structure of both representatives exhibits three crystallographically different Ln 3+ cations, each with a coordination number of eight. ( Ln 1) 3+ and ( Ln 2) 3+ are only coordinated by three oxygen atoms, whereas ( Ln 3) 3+ shows additional contacts to halide anions in forming square [ Ln O 4 X 4 ] 9– antiprisms. All As 3+ cations are surrounded by three oxygen atoms in the shape of isolated [AsO 3 ] 3– ψ 1 ‐tetrahedra. They occur either isolated or condensed as pyroanionic [As 2 O 5 ] 4– units with a bridging oxygen atom. In both anions, non‐binding lone‐pair electrons are present at the As 3+ cations with a pronounced stereochemically active function.