Triphenylphosphine oxide complexes of actinide tetrahalides. Crystal and molecular structure of trans-tetrabromobis(triphenylphosphine oxide)uranium(IV)

Abstract

The crystal structure of the title compound has been determined from three-dimensional X-ray diffraction data. The compound crystallises in space group P2₁ with a= 12.073(6), b= 15.824(7), c= 10.162(5)Å, β= 100.3(1)°, and Z= 2. The structure was solved by Patterson and Fourier methods; full-matrix least-squares refinement based on 1 836 observed reflections gave a final R value of 0.054 (R′= 0.048). There is a distorted octahedral array of ligands around the uranium atom with the triphenylphosphine oxide (tppo) molecules co-ordinated in trans positions. The U–O distances are 2.21(3) and 2.24(2)Å, and the U–Br distances range from 2.758(5) to 2.800(5)Å. A new crystal modification of tetrachlorobis(triphenylphosphine oxide)uranium (IV), β-UCl₄·2tppo, is reported. The structural data are discussed in conjunction with i.r. results for this metastable phase, for α-UCl₄·2tppo, and for other actinide complexes of the type MX₄·2tppo (M = Th–Pu inclusive; X = Cl or Br). It is concluded that the β-phase is the trans isomer. Solid-state electronic spectra are reported for α- and β-UCl₄·2tppo and UBr₄·2tppo.