The polyoxo rare-earth core Ln(8)(mu(4)-O)(mu(3)-OH)(12)(10+)(Ln = Y, Gd, and Yb) has been,synthesised from the appropriate rare-earth chloride hydrate and K2Se and Se in dmf (dimethylformamide). The cluster core is ligated with a variety of polyselenido chains in addition to a number of dmf molecules. The structure of the Gd-8(dmf)(13)(mu(3)-OH)(12)(Se-3)(Se-4)(2)(Se-5)(2) cluster, 1, was determined by X-ray diffraction methods. It is similar to an Eu cluster previously characterized. Two new clusters, Yb-8(dmf)(11)(mu(4)-OH)(mu(3)-OH)(12)(Se-4)(2)(Se-5)(2)Cl-2 . dmf, 2, and Y-8(dmf)(12)(mu(4)-O)(mu(3)-OH)(12)(Se-4)(4)Cl-2 . 6 dmf, 3, have also been synthesized and characterized. Clusters 2 and 3 have the same octanuclear core of rare-earth atoms as the Gd cluster but contain two chloro ligands in two isomeric conformations in place of the Se-3(2-) ring in the Gd cluster. The geometry of the Ln(8), core is described as a triangulated dodecahedron with mu(3)-OH groups capping the 12 faces. A mu(4)-O atom centers the cluster with close contacts to four Ln atoms in an approximate tetrahedral arrangement, Pertinent crystallographic data are: Compound I, monoclinic, C-2h(5)-P2(1)/n,a = 14.410(3) Angstrom, b = 24.439(5) Angstrom, c = 28.927(6) Angstrom, beta = 101.05(3)(0), V = 9998(3) Angstrom(3), T = 106(2) K, Z = 4; Compound 2, orthorhombic, D-2h(15)-Pbca, a = 17.049(9) Angstrom, b = 24.68(1) Angstrom, c = 45.03(2) Angstrom, V = 18,945( 16) Angstrom(3), T = 153(2) K, Z = 8; Compound 3, monoclinic, C-2h(5)-P2(1)/c, a = 18.728(1) Angstrom, b = 29.263(1) Angstrom, c = 20.548(1) Angstrom, beta = 90.144(1)(0), V = 11,261(1) Angstrom(3), T = 153(2) K, Z = 4.
