Vibrational study of (eta(5)-cyclopentadienyl) metal complexes

Raman and infrared spectral data on the vibrational modes of the M-(eta(5)-C5H5) unit have been collected and the fundamental assignments revisited. Complexes belonging to the metallocene series M(eta(5)-C5H5) and the cyclopentadienide salts [eta(5)-C5H5-][M(+)] have been studied, together with some monometallic (eta(5)-C5H5)ML, and bimetallic (eta(5)-C5H5)(2)M(2)L(2n) compounds. Comparison of the spectra shows a common vibrational pattern of the intraring modes, with minor variations in frequency and intensity, which allows the establishment of a vibrational fingerprint of general validity. Raman spectra give rise to a firm assignment of the skeletal metal-ring modes; a correlation is noted between the metal-ring stretch force constant and the metal-carbon distance. The approximate CH out-of-plane force constants vary considerably and presumably reflect different H-ring electrostatic repulsions.