The gas-phase structure of chloroferrocene from microwave spectra

Rotational spectra for ten isotopomers of chloroferrocene were measured using pulsed-beam Fourier transform microwave spectroscopy. Rotational transitions due to both "a" and "b" dipole moments were measured in the 4-12 GHz range. Thirty rotational constants were determined from microwave data for the normal, ⁵⁴Fe, ⁵⁷Fe, ³⁷Cl, and six unique ¹³C isotopomers, by fitting the microwave data using a rigid rotor Hamiltonian with centrifugal distortion and quadrupole coupling terms. The moments of inertia of the isotopomers were used in Kraitchman and in least-squares fitting analyses to determine gas-phase structural parameters. The unsubstituted cyclopentadienyl (C₅H₅) ligand was determined to have essentially fivefold local symmetry, while the chlorinated cyclopentadienyl ligand shows small, but significant distortions from the fivefold symmetry. The C-Cl bond is bent 2.7(6)° from the plane of carbon atoms. This substituted C₅H₄Cl ligand is tilted 1.4(5)° with respect to the unsubstituted C₅H₅ ligand. The carbon atoms of the two cyclopentadienyl ligands are eclipsed, similar to normal ferrocene. The average Fe-C distance is 2.042(9) Å. The estimated displacement of C-H bonds out of the planes of the carbon atoms is 4(2)° away from the metal atom.