The crystal structure of UCp3Cl(Cp-C5H5) by neutron diffraction

The structure of UCp3Cl contains both U-Cl σ-bonds and U-Cp π-bonds and thus represents a very interesting system to study in detail possible covalency of these bonds. Such experiments can be performed with polarized neutrons by measuring the spin density if precise structural parameters are known. We have therefore performed a neutron experiment to complement previous X-ray experiments, since the latter are not very sensitive to the C and H atoms in the presence of uranium. A low resolution model showed that a disorder model of Cp rings is necessary. The disorder of the Cp rings as viewed by the neutron diffraction is of a simple form with only two possibilities, ABC and A'B'C' rotated by 36 degrees. Space considerations show that the arrangements of all three Cp rings around one particular U atom must be of the same form (ABC or A'B'C'); RW = 6 %; χ about 1.5. At temperatures between 80 and 100 K, we have discovered a crystallographic phase transition in UCp3Cl. We have not fully characterized this yet since it results in a large deformation of the crystal, presumably due to internal strains. Tentatively we can make the following comments: (1) the transition leaves unchanged the crystallographic parameters (x,y,z) of the uranium and chlorine atoms and of the centers of the Cp rings (α, β, γ), (2) the transition is probably an ordering of the Cp rings as it is not observed in magnetic susceptibility measurements. (author) 5 refs., 1 fig., 1 tab