Abstract: | The crystal structures of Ce2[SeO3]3 and Pr2[SeO3]3 have been refined from X‐ray single‐crystal diffraction data. The compounds were obtained using stoichiometric mixtures of CeO2, SeO2, Ce, and CeCl3 (molar ratio 3:3:1:1) or Pr6O11, SeO2, Pr, and PrCl3 (molar ratio 3:27:1:2) heated in evacuated sealed silica tubes at 830 °C for one week. Ce2[SeO3]3 crystallizes orthorhombically (space group: Pnma), with four formula units per unit cell of the dimensions a = 839.23(5) pm, b = 1421.12(9) pm, and c = 704.58(4) pm. Its structure contains only a single crystallographically unique Ce3+ cation in tenfold coordination with oxygen atoms arranged as single‐face bicapped square antiprism and two different trigonal pyramidal [SeO3]2? groups. The connectivity among the [CeO10] polyhedra results in infinite sheets of face‐ and edge‐sharing units propagating normal to [001]. Pr2[SeO3]3 is monoclinic (space group: P21/n) with twelve formula units per unit cell of the dimensions a = 1683.76(9) pm, b = 705.38(4) pm, c = 2167.19(12) pm, and β = 102.063(7)°. Its structure exhibits six crystallographically distinct Pr3+ cations in nine‐ and tenfold coordination with oxygen atoms forming distorted capped square antiprisms or prisms (CN = 9), bicapped square antiprisms and tetracapped trigonal prisms (CN = 10), respectively. The [PrO9] and [PrO10] polyhedra form double layers parallel to (111) by edge‐ or face‐sharing, which are linked by nine different [SeO3]2? groups to build up a three‐dimensional framework. In both compounds, the discrete [SeO3]2? anions (d(Se4+–O2?) = 166–174 pm) show the typical Ψ1‐tetrahedral shape owing to the non‐bonding “lone‐pair” electrons at the central selenium(IV) particles. Moreover, their stereochemical “lone‐pair” activity seems to flock together in large empty channels running along [010] in the orthorhombic Ce2[SeO3]3 and along [101] in the monoclinic Pr2[SeO3]3 structure, respectively. |