A new interpretation of the disordered crystal structure of ferrocene

X-ray analyses of ferrocene [Fe(CsHs) 2, CmH~0Fe] at room temperature and at 173 K yield vibrationalellipsoid patterns that are incompatible with pure rotational disorder of the cyclopentadienyl rings. Below 164 K ferrocene becomes triclinic. Relationships between the diffraction patterns of the monoclinic hightemperature (HT) and triclinic low-temperature (LT) structures suggest that the formally centrosymmetric molecule of the disordered HT phase can be described in good approximation as an averaged superposition of the four molecules present in the primitive LT cell. The molecular centre of symmetry required by the HT space group is thus only statistical in nature, and the crystallographic evidence for the staggered arrangement of the cyclopentadienyl rings in ferrocene and other isomorphous metallocenes has to be revised.