New complexes of general formula [U(q-C,H,),(OR),][R = Et (1). Pri (2), or But (3)] have been prepared by reaction of [U(-q-C,H,),] with ROH. The crystal structure of (2) has been determined from single-crystal X-ray diffraction data (Mo-K, radiation). The complex crystallizes in the space group P2Jc with unit-cell dimensions a = 12.324(7), b = 8.535(1), c = 17.897(9) A, = 126.71 (3)". Z = 4, and R 0.030 for 1 408 independent observations. In the dimeric molecular structure, with 7 crystallographic site symmetry, two uranium atoms are joined by two alkoxide bridges. The co-ordination polyhedron of the uranium can be regarded as a distorted pentagonal bipyramid, whose equatorial plane is defined by the terminal carbons of the 3-ally1 ligands and by a bridging oxygen. The axial positions of the bipyramid are occupied by the remaining bridging oxygen and by a terminal oxygen. The latter shows a linear co-ordination [U-0-C 178.0( 1 O)"]. Other relevant bond distances are : U-O(bridging) 2.271 (10) and 2.41 3(10) ; U-O(termina1) 2.056(13) ; and U-C(mean) 2.679( 14) A. Infrared data for (1)-( 3) suggest that the uranium-ally1 x bond is predominantly ionic; this is consistent with some structural peculiarities of the allyl ligands in (2). Hydrogen-1 n.m.r. results indicate monomeric structures in tetrahydrofuran [(1)-( 3)] and dimeric ones in toluene solution for (1) and ( 2).THE synthesis of tetra-allyluranium [U(-q-C,H,)J, and its performance as a catalyst for the homogeneous polymerization of diolefins, have been previously re-p0rted.l Uranium alkoxides, however, more readily available and more stable than the tetra-allyluranium derivatives, in combination with AlR, and a Lewis acid give very active and highly stereospecific catalysts., In this case, we believe that a q-ally1 intermediate is formed ' in s i t N ' when the catalyst is prepared in the presence of This feature seems to be peculiar to uranium organo-10-15y0. metallic c o m p o ~n d s . ~
