Barriers to rotation of the cyclopentadienyl ligand: spin-lattice relaxation time measurements and atom–atom potential calculations on cyclopentadienyl manganese and rhenium tricarbonyl and vanadium tetracarbonyl complexes

The barriers to cyclopentadienyl ring rotation in the solid phase have been measured by spin-lattice relaxation time methods for the organometallic complexes CpMn(CO)₃ (7.24 kJ mol⁻¹), CpRe(CO)₃ (7.15 kJ mol⁻¹), and CpV(CO)₄ (7.07 kJ mol⁻¹), where Cp = η⁵-C₅H₅. Nonbonded atom–atom potential calculations of the barriers in these complexes and in BzCr(CO)₃ (Bz = η⁶-C₆H₆) show that the molecular conformation of the Mn and Re compounds is determined by crystal packing forces and that concerted ring motions are possible for the cyclopentadienyl complexes, but not for the benzene chromium tricarbonyl.