Structure and stereochemistry in f-block complexes of high co-ordination number. Part 4. The [M(bidentate ligand)4] system : crystal structures of tetrakis(dibenzoylmethanato)-uranium(IV) and -cerium(IV)

Abstract

The crystal structure of [U^IV(PhCOCHCOPh)₄] has been determined at 295 K from X-ray diffraction data refined by least squares to a residual of 0.042 for 2 257 ‘observed’ reflections. Crystals are orthorhombic, space group Pccn, with unit-cell dimensions a= 10.323(8), b= 20.172(15), c= 23.65(2)Å, and Z= 4; they are isostructural with the previously studied cerium analogue, the eight-co-ordinate uranium atom lying on a crystallographic two-fold axis, with U–O ranging from 2.323(7) to 2.370(6)Å. For the cerium analogue, a= 10.29(1), b= 20.07(3), and c= 23.42(4)Å; R= 0.043 for 1 817 ‘observed’ reflections. The corresponding Ce–O distances lie between 2.291 (7) and 2.348(6)Å. The stereochemistries of these complexes are close to square antiprismatic with the bidentate ligands spanning opposite square faces, as predicted by electron-pair repulsion theory.