(untitled) - Eiland, P F; Pepinsky, R (J Am Chem Soc, 1952)
Two symmetrically independent molecules are present in the unit cell for ferrocene heterocycle 3 (CCDC-2477791); the molecule not shown in Fig. 2 is slightly less distorted, i.e., 2.41° between the two Cp ring planes and 6.58° between Cp and benzene ring planes in the fused tricyclic system. Non-parallel Cp ring planes within a ferrocene is not necessarily an indication of instability; e.g., XRD analysis of the acetylpyridinium hexafluoroantimonate derivative of ferrocene DMAP analogue 1 (CCDC-175230) shows a dihedral angle of 7.58° between least squares fitted planes defined by each Cp ring. XRD analysis of ferrocene (CCDC-1154857) shows perfect parallel alignment between Cp ring planes and a distance of 1.661Å from the Fe-atom to each Cp ring centroid. For seminal reports on single crystal XRD analysis of ferrocene, the first metallocene, see: