Hydrated hexa­cyano­metallate(III) salts of triaqua­(18-crown-6)lan­than­oid(III) and tetra­aqua­(18-crown-6)lanthanoid(III) cations containing nine- and ten-coordinate lanthanoids

Tetra­aqua­(18-crown-6)cerium(III) hexa­cyano­ferrate(III) di­hydrate, [Ce(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, and tetra­aqua­(18-crown-6)neodymium(III) hexa­cyano­ferrate(III) dihydrate, [Nd(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, are iso­morphous and isostructural in the C2/c space group, where the cations, which contain ten-coordinate lanthanoid centres, lie across twofold rotation axes and the anions lie across inversion centres. In these compounds, an extensive series of O—H...O and O—H...N hydrogen bonds links the components into a continuous three-dimensional framework. Triaqua­(18-crown-6)lanthanoid(III) hexa­cyano­ferrate(III) di­hydrate, [Ln(C₁₂H₂₄O₆)(H₂O)₃][Fe(CN)₆]·2H₂O, where Ln = Sm, Eu, Gd or Tb, are all isomorphous and isostructural in the P space group, as are triaqua­(18-crown-6)gadolinium(III) hexa­cyano­chromate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Cr(CN)₆]·2H₂O, and triaqua­(18-crown-6)gadolinium(III) hexa­cyano­cobaltate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Co(CN)₆]·2H₂O. In these compounds, there are two independent anions, both lying across inversion centres, and the lanthanoid centres exhibit nine-coordination; in the crystal structures, an extensive series of hydrogen bonds links the components into a three-dimensional framework.