Rigid-Body Disorder Models for the High-Temperature Phase of Ferrocene

Ferrocene, [Fe(C₅H₅)₂], which crystallizes at room temperature in space group P2₁/a with Z = 2, is described in many textbooks as having D5d symmetry. Previous work has shown, however, that the librational amplitude associated with motion about the fivefold axis does not decrease with temperature and that the crystals are probably disordered. Ferrocene molecules in triclinic crystals grown below 169 K have approximate D₅ symmetry and an almost eclipsed conformation; the low- and high-temperature phases may be related by an order-disorder transition, during which the number of independent atoms changes by a factor of 4. The structure of the high-temperature phase has been reinvestigated with rigid-body refinements of the neutron diffraction data collected at 173 and 298 K by Takusagawa & Koetzle [Acta Cryst. (1979), B35, 1074-1081]. The C₅H₅ ring was treated as a rigid group of C₅ symmetry; C-C and C-H distances were allowed to vary, as was the displacement of the H atoms from the C₅ plane. The rigid-body motion of the C₅H₅ ligand was described by the TLS model. All the rigid-body disorder models fit better than conventional independent-atom models. A disorder model that includes three sites for each C₅H₅ ring is the best of the models that were investigated, which indicates that the structure of the high-temperature phase cannot be described by the superposition of the two independent ferrocene molecules in the low-temperature phase. The phase transition between the high- and low-temperature phases is not a simple order-disorder transition.

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