sigma- vs. π-bonded organoactinides. The synthesis and structural analysis of tris(eta5-cyclopentadienyl)-eta1-2-methylallyluranium(IV)

The synthesis, characterization, and structural analysis of the title compound, (C5H5)3[CH3C(CH2)2]U/sup IV/, is reported. The reaction of U(C5H5)3Cl with Mg[CH3C(CH2)2]Cl under inert atmosphere conditions at - 780C in THF gives a high yield of the uranium allyl. Slow recrystallization gives dark red-brown crystals. The structure has been determined from three-dimensional x-ray diffraction data collected by counter methods. Three π-bonded eta5-cyclopentadienide rings are bonded to the uranium atom with an average bond distance of 2.74(1) A. A sigma-bonded eta1-2-methylallyl group completes the coordination sphere with a U--C bond length of 2.48(3) A. The coordination geometry is a distorted tetrahedron with approximate C/sub 3v/ symmetry. The average (center of cyclopentadienide ring)--U--(allyl carbon) bond angle is 1000. A sigma-bonded ground state geometry vs. the π-bonded geometry observed in (C5H5)4U is explained by the competition between the reorganization energy and the increase in coordinate bonds on going from the sigma- to π-bonded complexes. The crystals conform to space group P212121 with a = 10.240 (4), b = 8.747 (4), c = 18.198 (4) A, Z = 4, and rho/sub calcd/ = 1.99 g/cm3. A total of 3470 reflections were collected of which 1698 independent reflections with F2 greater than or equal to 3 sigma (F2) were used in the final refinement by full-matrix least-squares methods to give weighted and unweighted R factors of 6.4 and 6.5 percent, respectively. (U.S.)