Reorientational motions of permethylated cyclopentadienyl rings in polycrystalline organometallic compounds

Reorientational motions in the crystalline complexes (C5Me5)2Fe, (C5Me5)Rh(CO)2, (C5Me5)2Cr2(CO)4, (C5Me5)Fe2(mu2-CO)2(CO)2 and (C5Me5)2Rh2(mu2-Cl)2Cl2 were investigated via wide-line and CP-MAS NMR spectroscopy and atom-atom potential energy barrier calculations. The rotation of the methyl group about its C3 axis and the reorientation of the Cp ring about its C5 coordination axis have been determined. The potential barriers calculated from known crystal structures are comparable to the activation energies from 1H spin-lattice relaxation time measurements.