Structures of Substituted-Cyclopentadienyl Uranium(III) Dimers and Related Uranium Metallocenes Deduced by EXAFS

The crystal structures of [Cp‘ ‘2UF]2 and [Cp⧧ 2UO]2 are reported, where Cp‘ ‘ is 1,3-(Me3Si)2C5H3 and Cp⧧ is 1,3-(Me3C)2C5H3. Both complexes have idealized C 2 h symmetry, and their U···U distances are 3.85 and 3.39 Å, respectively. The X-ray absorption spectra of several uranium metallocene complexes, and the numerical results from fitting the EXAFS spectra, are reported. For [Cp⧧ 2UF]2, the U···U distance was found by EXAFS to be similar to that in [Cp‘ ‘2UF]2, implying that [Cp⧧ 2UF]2 is dimeric. A structural model 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.