Vibrational spectroscopic and force field studies of (η⁵-Cp)ML₃-type complexes (M=Mn, Re; L=CO, O)

High- and low-oxidation state CpML₃-type (M=Mn, Re; L=O, CO) cyclopentadienyl complexes have been investigated by vibrational spectroscopy (FTIR, FT-FIR, FT-Raman) and normal coordinate calculations. The vibrational spectra of CpMn(CO)₃, CpRe(CO)₃ complexes were revised and reinterpreted. For the oxo-complexes and Cp*-carbonyl compounds, Cp*Mn(CO)₃ and Cp*Re(CO)₃, a complete spectral assignment is proposed. The results of the normal coordinate analysis are in good agreement with the spectral evidence. The vibrational spectroscopic findings help to explain earlier observations, e.g. the significantly lower stability of CpReO₃ in comparison to Cp*ReO₃. Characteristic force constants have been determined for Cp and Cp* ligands. A method is described for estimating an approximate force constant for the metal cyclopentadienyl (Cp) ligand bond stretch in half sandwich type of complexes, based on the use of an effective 'spectroscopic' mass of the Cp-ligand.