The variable-temperature H-1 NMR behavior of the uranium(III) dimers [Cp"2U
X](2) and [(Cp2UX)-U-double dagger](2), where X is F, Cl, Br, or I, Cp" is
1,3-(Me3Si)(2)C5H3, and Cp-double dagger is 1,3-(Me3C)(2)C5H3, has been exa
mined. At low temperature, the number of inequivalent CMe3 or SiMe3 groups
implies that the solution structure is the same as the solid-state structur
e in all of these complexes. The barriers to ring rotation in the Cp" serie
s are strongly dependent upon the U-X distance, but all of the barriers to
ring rotation in the Cp" series are the same. The trends in ring rotation b
arriers are explained by the different conformations of the Cp ligands in t
he dimers. In addition to the homo-halide dimers, the variable-temperature
NMR, behavior of the hetero-halide dimers Cp'U-4(2)(mu-X)(mu-Y), where Cp'
is Cp" or Cp-double dagger and X and Y are halides where X not equal Y, was
examined. Above room temperature, the halide atoms exchange sites rapidly
on the NMR time scale.