Inelastic neutron scattering studies of benzene-containing organometallic complexes. Part 1. Complexes [Cr(CO)3(η6-C6H6)] and [ Mn(CO)3(η6-C6H6)]Br

Inelastic neutron scattering spectra (0–875 cm–1) of [Cr(CO)3(η6-C6H6)](1) and [Mn(CO)3(η6-C6H6)]Br (2) have been measured. In addition, low-frequency i.r. (60–600 cm–1) and Raman (0–l 000 cm–1) spectra of (2) are reported for the first time. Those low-frequency modes of both compounds which involve significant benzene-ligand displacement have been assigned. From the assignments of the benzene-ligand torsion (ν22) in complexes (1) and (2) to bands at 88.5 and 116.5 cm–1 respectively, values for the activation energy for benzene-group rotation have been calculated to be 27.5 and 46.5 kJ mol–1 respectively. In the case of complex (1) this value has been compared with that determined from n.m.r. measurements (17.6 kJ mol–1).