Syntheses and Crystal Structures of Rb 2 B 2 Se 7 , Tl 2 B 2 Se 7 , Cs 3 B 3 Se 10 , and Tl 3 B 3 Se 10 : New Perseleno‐selenoborates with Polymeric Anionic Chains

The perseleno‐selenoborates Rb 2 B 2 Se 7 and Cs 3 B 3 Se 10 were prepared from the metal selenides, amorphous boron and selenium, the thallium perseleno‐selenoborates Tl 2 B 2 Se 7 and Tl 3 B 3 Se 10 directly from the elements in evacuated carbon coated silica tubes by solid state reactions at temperatures between 920 K and 950 K. All structures were refined from single crystal X‐ray diffraction data. The isotypic perseleno‐selenoborates Rb 2 B 2 Se 7 and Tl 2 B 2 Se 7 crystallize in the monoclinic space group I 2/a (No. 15) with lattice parameters a = 12.414(3) Å, b = 7.314(2) Å, c = 14.092(3) Å, β = 107.30(3)°, and Z = 4 for Rb 2 B 2 Se 7 and a = 11.878(2) Å, b = 7.091(2) Å, c = 13.998(3) Å, β = 108.37(3)° with Z = 4 for Tl 2 B 2 Se 7 . The isotypic perseleno‐selenoborates Cs 3 B 3 Se 10 and Tl 3 B 3 Se 10 crystallize in the triclinic space group P1 (Cs 3 B 3 Se 10 : a = 7.583(2) Å, b = 8.464(2) Å, c = 15.276(3) Å, α = 107.03(3)°, β = 89.29(3)°, γ = 101.19(3)°, Z = 2, (non‐conventional setting); Tl 3 B 3 Se 10 : a = 7.099(2) Å, b = 8.072(2) Å, c = 14.545(3) Å, α = 105.24(3)°, β = 95.82(3)°, γ = 92.79(3)°, and Z = 2). All crystal structures contain polymeric anionic chains of composition ([B 2 Se 7 ] 2– ) n or ([B 3 Se 10 ] 3– ) n formed by spirocyclically fused non‐planar five‐membered B 2 Se 3 rings and six‐membered B 2 Se 4 rings in a molar ratio of 1 : 1 or 2 : 1, respectively. All boron atoms have tetrahedral coordination with corner‐sharing BSe 4 tetrahedra additionally connected via Se–Se bridges. The cations are situated between three polymeric anionic chains leading to a nine‐fold coordination of the rubidium and thallium cations by selenium in M 2 B 2 Se 7 (M = Rb, Tl). Coordination numbers of Cs + (Tl + ) in Cs 3 B 3 Se 10 (Tl 3 B 3 Se 10 ) are 12(11) and 11(9).