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

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, ν4ip, ν4op, ν6op, and ν16ip, 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).