A series of low water content cerium(III) squarates were produced by pseudo-hydrothermally treating the compound Ce2(H2O)11(C4O4)3·2H2O (1-Ce) in water in a closed vessel. They are: [Ce(H2O)2]2(C4O4)3 (1a-Ce), [Ce(H2O)2]4(C4O4)6·3H2O (1b-Ce) and [Ce,Ce(H2O)2]2(OH)2(C4O4)5·0.6H2O (1c-Ce). The crystal structures were determined by X-ray single crystal techniques, 1a-Ce: monoclinic, space group P21/c, a = 7.143(1), b = 16.940(4), c = 6.994(1) Å, β = 101.24(2)°, V = 830 Å3, Z = 2; 1b-Ce: triclinic, space group P1, a = 7.579(1), b = 8.921(2), c = 7.054(1) Å, α = 105.06(2), β = 94.02(1), γ = 89.94(2)°, V = 459 Å3, Z = 1; 1c-Ce: triclinic, space group P1, a = 8.066(1), b = 12.992(1), c = 7.100(2) Å, α = 96.96(1), β = 105.15(1), γ = 105.93(1)°, V = 676 Å3, Z = 1. The squarate anion presents various unusual coordination modes, including chelation, resulting in cerium-oxygen-cerium double bridges, 1a-Ce has a layer structure, 1b-Ce a tunnel structure and 1c-Ce, that further contains cerium-hydroxo-cerium bridges, has a dense 3-D structure. The structure type 1a-Ce has been shown to exist also for La, Pr, Nd, Sm and Eu. The thermal behaviours of 1a-La, 1a-Ce, 1a-Pr, 1a-Nd and 1a-Sm were studied. © 1990.
