Inelastic neutron scattering studies of benzene-containing organometallic complexes. Part 2. Complexes [Cr(η6-C6H6)2] and [Cr(η6-C6H6)2]I

Abstract

Inelastic neutron scattering spectra (0–875 cm^–1) of [Cr(η⁶-C₆H₆)₂] and [Cr(η⁶-C₆H₆)₂] I have been measured. From the spectroscopic data it has been possible firmly to assign those low-frequency modes of both compounds which involve significant displacement of the benzene ligands. In addition, the optically inactive internal modes of the benzene ligands, ν₄^ip, ν₄^op, ν₆^op, and ν₁₆^ip, have been observed. Employing the values of 59.5 and 66.0 cm^–1 assigned to the torsional mode of the benzene ligands (ν₂₂) in [Cr(η⁶-C₆H₆)₂] and [Cr(η⁶-C₆H₆)₂]I, values of 12.6 and 15.5 kJ mol^–1 respectively have been calculated for the barriers to benzene-group rotation. In the case of [Cr(η⁶-C₆H₆)₂], this value has been compared with the activation energy determined from n.m.r. measurements (19.1 kJ mol^–1).