Structure and bonding in octahedral uranium(IV) complexes of the type UX4·2L (X = halogen, L = unidentate, neutral oxygen donor). Part 2. The crystal structures of tetrachlorobis[tris(pyrrolidinyl)phosphine oxide]-, tetrachlorobis(di-isobutyl sulphoxide)-, and tetrabromobis(triphenylarsine oxide)-uranium(IV)

Abstract

The crystal structures of three complexes of uranium(IV) halides, UCl₄·2tprpo (1), UCl₄·2dibso (2), and UBr₄·2tpao (3)[tprpo = tris(pyrrolidinyl)phosphine oxide, dibso = di-isobutyl sulphoxide, and tpao = triphenylarsine oxide], have been determined from three-dimensional X-ray diffraction data, and refined by full-matrix least-squares methods. For (1) the crystals are monoclinic, space group C2/c(final R= 0.046), for (2) monoclinic, space group P2₁/c(final R= 0.047), and for (3) monoclinic, space group P2₁(final R= 0.080). In each complex the uranium co-ordination is octahedral, with slight displacements (0.1 Å) of the bromine atoms in (3) from their mean plane. The molecules in (1) and (2) have inversion symmetry. Compound (3) is isomorphous with the triphenylphosphine oxide analogue, UBr₄·2tppo; the molecule is devoid of symmetry and adopts the same conformation as in the isomorph. The U–Cl distances are 2.620(4)Å in (1) and 2.605(3)Å in (2), while U–Br is 2.803(7)Å in (3); U–O is 2.233(7)Å in (1), 2.248(7)Å in (2), and 2.12(3)Å in (3). The U–X (X = halogen) and U–O bond lengths obtained in this work and in Part 1 of this series are compared with other estimates of these bonds in similar complexes. It is found (i) that the inverse relationship between the bond-length changes δ(U–X) and δ(U–O) previously noted for complexes containing the UO₂X₄ chromophore (‘axial–equatorial effect’) is maintained in the complexes studied here, thereby providing further evidence for the ligand–ligand repulsion effects which dominate the (largely ionic) bonding in such complexes, and (ii) that the data, when grouped according to generic ligand type, lend support for the donor-strength sequence AsO > PO > SO > CO based on chemical evidence.