Vibrational Study of (η5-Cyclopentadienyl)metal Complexes

Raman and infrared spectral data on the vibrational modes of the M−(η5-C5H5) unit have been collected and the fundamental assignments revisited. Complexes belonging to the metallocene series M(η5-C5H5) and the cyclopentadienide salts [η5-C5H5-][M+] have been studied, together with some monometallic (η5-C5H5)MLn and bimetallic (η5-C5H5)2M2L2n 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.