The molecular structure, equilibrium conformation and barrier to internal rotation in decachloroferrocene, Fe(η-C5Cl5)2, determined by gas electron diffraction

The molecular structure of decachloroferrocene has been determined by gas electron diffraction supported by quantum chemical calculations. The equilibrium conformation has staggered ligand rings and D5d symmetry. The barrier to internal rotation is, however, only 0.8(2) kJ mol−1. This barrier is so low that even at room temperature the vast majority of molecules in the gas phase would have sufficient thermal energy to undergo virtually non-hindered internal rotation. While the eclipsed equilibrium conformation of unsubstituted ferrocene is determined by attractive dispersion interaction between the two cyclopentadienyl ligands, the staggered equilibrium conformation of Fe(η-C5Cl5)2 is due to steric repulsion between Cl atoms at different rings. The ligands are non-planar: the C–Cl bonds are bent 3.7(3)° out of the plane of the C5 ring away from the metal atom. The Fe–C, C–C and C–Cl bond distances (ra) are: 205.0(4) pm, 143.4(3) pm and 170.2(4) pm respectively.