NMR studies of the reorientational motions of cyclopentadienyl ligands in solid cis‐ and trans‐[(η⁵‐C₅H₅)₂Fe₂(CO)₂(μ₂‐CO)₂]

Chemical shift anisotropies and spin‐lattice relaxation times, T₁, were determined for bridging and terminal carbonyls in c's and trans isomers of [(η⁵‐C₅H₅)₂Fe₂(CO) ₂(μ₂‐CO)₂]. The relaxation path of the ¹³CO resonances occurs through the dipolar interaction with the cyclopentadienyl protons modulated by the reorientational motion of the C₅H₅ (Cp) rings around their five‐fold axes. Wide‐line proton spin‐lattice relaxation times, T₁, at variable temperature allow the evaluation of the activation energies associated with the internal rotation of the cyclopentadienyl rings. In the cis isomer, the two crystallographically non‐equivalent Cp ligands rotate at different rates (E_a = 7.2 and 15.8 kJ mol⁻¹, respectively). Carbon‐13 spin‐lattice relaxation times in the rotating frame of the cyclopentadienyl carbons of the cis isomer support this view. Finally, good agreement is found between our work, the results obtained from crystallographic data and theoretical calculations on the energy barriers for the rotation of the Cp rings.