The crystal structure of [UO,CI2(PPh30),] has been determined from three-dimensional X-ray diffraction data.Crystals are triclinic, space group P i , with a = 10.0101 (6). b = 10.2589(9), c = 9.2347(8) A, a = 110.093(6), p = 92.1 29(6), and y = 78.384(6)", and Z = 1. The structure has been solved by the heavy-atom method from counter data, and refined by least squares to a final R of 0.054. The co-ordination polyhedron around uranium is a distorted octahedron, with a linear uranyl group (U-0 1.764 8 ) perpendicular to a plane in which the two chloride and two oxide ions trans to each other occupy the corners of a rectangle (U-0 2.300; U-CI 2.645 A).THE uranium(v1) compounds [UO,X,(PR,O),] (X = C1or Br; R = Me, E t , or Ph) were reported by Gans and Smith and further studied by Day and Venanzi who also prepared the iodo-compound (X = I). The compounds would be expected to contain a linear uranyl group (0-U-0) with the four other ligands approximately in the equatorial plane. From i.r. studies, and the general solubility of the compounds in low-polarity organic solvents, it was suggested2 that the ligands adopt a t r a m configuration. However, since the (analytically isomeric) ionic uranium(v1) compounds, [U02( PPh,O),]-[UO,X,] were also prepared,2 in which the cation presumably contains four triphenylphosphine oxide ligands co-ordinated in or near the equatorial plane, a cis configuration for [UO,Cl,(PPh,O),] could not be ruled out on steric grounds. Furthermore, the related uraniurn(1v) complex, [UCl,(PPh,O).j, has recently been shown to contain cis-PPh,O groups, in a pseudo-octahedral structure. It was of interest, therefore, to investigate the structure of [UC),Cl,(PPh,0)2], which is reported in this paper. EXPERIMENTAL Dichlorodioxobis (triphenylphosphine oxide) uranium (VI) was prepared by stoicheiometric addition of uranyl chloride (UCl,O,) and triphenylphosphine oxide in ethanol. Clear yellow crystals suitable for the single-crystal X-ray analysis were obtained by recrystallization from ethanol.The crystal used was a prism with dimensions ca. 0.1 x 0.1 x 0.2 mm. All the X-ray experiments were performed using a single-crystal Philips PW-1 100 diff ractometer and graphite-monochromated Mo-K, radiation ( A 0.7 10 7 A).The triclinic unit cell was determined on the basis of 15 reflections found by mounting the crystal a t random and varying the orientation angles #I and x in ranges of 90 and 45", respectively, with the detector position varying between 0 = 6 and 12".For the determination of precise lattice parameters, 47 reflections having 6 < 0 < 12" were considered, and the precise diffraction angles 0 were evaluated as centres of gravity of their profiles I = f ( 0 ) , averaging over positive 0 values.Integrated intensities for hkl reflections with I > 0 and 1