Vibrational Overtone Spectroscopy of Pi-Bonded Organometallic Complexes

The vibrational overtone spectra of gaseous butadiene iron tricarbonyl (BDIT), benzene chromium tricarbonyl (BCT), cycloheptatriene chromium tricarbonyl (CHTCT), and cycloocatetraene iron tricarbonyl (COTTT) were recorded at the third overtone region using photoacoustic spectroscopy. These spectra were compared to the spectra of 1,3-butadiene, benzene, cycloheptarriene and cyclooctatraene, respectively. The third overtone spectrum of BDIT shows three transitions corresponding to the three different C-H local mode oscillators, all all shifted to the red compared to the corresponding peaks of 1,3-butadiene. No changes were observed in the overtone spectra of BCT and COTTT compared to the spectra of the corresponding hydrocarbon ligands. However, the third overtone spectrum of CHTCT contains two additional peaks. The presence of these additional transitions suggested vibrational coupling of the C-H stretch with other motions of the hydrocarbon ring bonded to the transition metal.