Lanthanoid(III) oxide chloride oxoselenates(IV): A fascinating class of multinary compounds

Compounds in the systems M3+-O2−-Cl−-Se4+ are more than just quaternaries, for they can offer both oxygen that is bonded to selenium(IV) to form discrete [SeO3]2− ψ1-tetrahedra ([SeO3E]2− with E: non-bonding electron pair as pseudo-ligand) and oxygen that is only bonded to the trivalent lanthanoid (M = Ce-Lu) to form regular [OM4]10+ tetrahedra. The latter share edges and vertices in most cases to build up cationic chains ([OM2]4+ in Cl−-free Tb2O[SeO3]2: a = 1064.57(7) pm, c = 521.56(4) pm, Z = 4, tetragonal, P42/ncm), double strands ([O2M3]5+ in Tb3O2Cl[SeO3]2: a = 535.16(4) pm, b = 1530.51(9) pm, c = 1081.72(7) pm, Z = 4, orthorhombic, Pnma; Er3O2Cl[SeO3]2: a = 1498.23(9) pm, b = 1102.04(7) pm, c = 547.95(4) pm, β = 105.515(6)°, Z = 4, monoclinic, C2/c) or layers ([O3M4]6+ in Yb4O3Cl2[SeO3]2: a = 853.87(6) pm, b = 1145.92(8) pm, c = 1195.46(8) pm, α = 68.132(6)°, β = 78.113(6)°, γ = 85.748(6)°, Z = 2, triclinic, P1¯, [O4M5]7+ in Tb5O4Cl3[SeO3]2: a = 1229.13(8) pm, b = 546.17(4) pm, c = 978.79(6) pm, β = 90.485(6)°, Z = 2, monoclinic, C2/m; [O8M9]11+ in Gd9O8Cl3[SeO3]4: a = 680.17(5) pm, b = 882.86(7) pm, c = 960.18(7) pm, α = 97.241(6)°, β = 95.539(6)°, γ = 96.265(6)°, Z = 1, triclinic, P1¯; [O3M3]3+ in CsEu4O3Cl3[SeO3]2: a = 545.09(4) pm, b = 879.98(7) pm, c = 1597.45(9) pm, α = 81.036(6)°, β = 89.992(6)°, γ = 71.925(6)°, Z = 2, triclinic, P1¯). On the other hand and much like in the pure lanthanoid(III) oxoselenates(IV) M2[SeO3]3 themselves, segregation of the stereochemically active non-bonding electron pairs (one per Se4+ cation) of the pyramidal [SeO3]2− units occurs, such that “lone-pair” channel or layer structures emerge. Sometimes even the “lone-pair” electrons of the chloride anions help to construct these structural features as also observed in the O2−-free chloride oxoselenates(IV) MCl[SeO3].