Structures of substituted-cyclopentadienyl uranium(III) dimers and relateduranium metallocenes deduced by EXAFS

The crystal structures of [Cp"2UF](2) and [(Cp2UO)-U-double dagger](2) are reported, where Cp" is 1,3-(Me3Si)(2)C5H3 and Cp-double dagger is 1,3-(Me3C )(2)C5H3, Both complexes have idealized Cah symmetry, and their U ... U dis tances are 3.85 and 3.39 Angstrom, respectively. The X-ray absorption spect ra of several uranium metallocene complexes, and the numerical results from fitting the EXAFS spectra, are reported. For [(Cp2UF)-U-double dagger](2), the U ... U distance was found by EXAFS to be similar to that in [Cp"2UF]( 2), implying that [(Cp2UF)-U-double dagger](2) is dimeric. A structural mod el is advanced that correlates the U ... U distance with the orientation of the cyclopentadienyl rings; the orientation is due to a subtle combination of steric repulsions between ligands on the same metal center and between ligands on adjacent metal centers.