Molecular structure of dimeric iron trichloride in the vapour phase as determined by electron diffraction

The molecular structure of the title compound has been determined by gas-phase electron diffraction, the vapour composition being obtained by quadrupole mass spectrometry. The experimental data can best be approximated by a model with a puckered four-membered ring (C2v symmetry). This deviation from D2h symmetry may be a consequence of large-amplitude vibrations around an imaginary axis connecting the two bridging chlorine atoms. The bond lengths and angles for the puckered-ring model are ra(Fe–Cl)(terminal) 2.127(4), ra(Fe–Cl)(bridge) 2.326(5)Å, Clt–Fe–Clt 124.3(7), and Clb–Fe–Clb 90.7(4)°. The bond lengths and force constants indicate that the terminal Fe–Cl bonds are considerably stronger than the bridging ones.