U(IV)/Ln(III) mixed site in polymetallic oxalato complexes. Part III: Structure of Na[Yb(C2O4)2(H2O)].3H2O and the derived quadratic series (NH4+)1-x[Ln1-xUx (C2O4)2(H2O)].(3+x) H2O, Ln=Y, Pr-Sm, Gd, Tb

Single crystals of a new sodium ytterbium oxalate Na[Yb(C2O4)2(H2O)].3H2O (1) and six mixed lanthanide (III)-uranium (IV) oxalates (NH4+)1-x[Ln1-xUx (C2O4)2(H2O)].(3+x) H2O, Ln=Y, x=0.47 (2), Ln=Pr, x=0.42 (3), Ln=Nd, x=0.60 (4), Ln=Sm, x=0.55 (5), Ln=Gd, x=0.25 (6) and Ln=Tb, x=0.52 (7) have been grown using slow diffusion through silica gels. The crystal structures of all the compounds have been determined by single-crystal X-ray diffraction. For compound 1 the symmetry is monoclinic, space group Pc, cell dimensions a=8.559(2)A, b=8.564(2)A, c=14.938(3)A, β=103.062(3), Z=4. The structure of 1 is isotypic with Na[Y(C2O4)2(H2O)].3H2O and consists of layers formed by four-membered rings of Yb connected through oxalate ions. The ytterbium atom is nine-coordinated by oxygen from four bis-bidentate oxalate ligands and one water molecule which alternate up and down the layer. Na+ ions and supplementary water molecules are located between the layers. The six mixed lanthanide (III)-uranium (IV) oxalates, 2-7, are isotypic, the symmetry is tetragonal, space group P4/n, the unit cell parameters are in the range 8.7239(12)-8.9116(6) and 7.854(2)-7.9487(9)A for a and c, respectively, Z=2. The structure of the six compounds is built from the same two-dimensional arrangement of alternating metallic and oxalate ions forming four-membered rings. The layers are similar to that observed in 1 and the mixed Ln(III)/U(IV) oxalate layers are obtained by partial substitution of Ln(III) by U(IV) in a nine-coordinated site, the charge surplus being compensated by removal of monovalent cations in the interlayer space. The ammonium ions and the water molecules are disordered in the same crystallographic site. Thus these compounds form the third series of mixed lanthanide (III)-uranium (IV) oxalates, the tetragonal one, that completes the two others previously reported, the hexagonal and the triclinic series