Preparation, solution behavior, and solid-state structures of (1,3-R2C5H3)(2)UX2, where R is CMe3 or SiMe3 and X is a one-electron ligand

High-yield preparations of the uranium metallocenes [1,3-(Me3Si)(2)C5H3](2) UCl2 (Cp "2UCl2) and [1,3-(Me3C)(2)C5H3](2)UCl2 (Cp double dagger(2)UCl(2)) have been developed from the reaction of UCl4 and the corresponding magnes ocenes Cp "Mg-2 and Cp double dagger(2)Mg in diethyl ether. The chloride li gands can be exchanged with either Me3SiBr or Me3SiI to give the uranium me tallocene bromides or iodides. The fluorides were prepared by the reaction of BF3. OEt2 with Cp "U-2(NMe)(2), Cp double dagger(2)U(OMe)(2), or Cp doub le dagger(2)UMe(2). The crystal, structures of Cp "2UCl2, Cp double dagger( 2)UCl(2), Cp "2UMe2, Cp double dagger(2)UF(2), and dimeric (Cp "2UF2)(2) ar e reported. The idealized symmetry of the monomers Cp "2UX2 and Cp double d agger(2)UCl(2) is C-2v when X is F, Cl, or Br and C-2 when X is I or Me; in the dimer, the idealized symmetry is C-i. This preference is rationalized by intramolecular and intermolecular steric effects. The solution ring conf ormations and intramolecular exchange processes have been studied by variab le-temperature H-1 NMR spectroscopy. In all cases, the low-temperature limi ting spectra are consistent with the idealized symmetry observed in the sol id state.