Vibrational Overtone Spectroscopy of Butadiene Iron Tricarbonyl and 1,3-Butadiene

The photoacoustic vibrational overtone spectrum of gaseous butadiene iron tricarbonyl (BDIT) at the third overtone region was recorded and compared to that of 1,3-butadiene. The effect of the metal, determined by an interpretation of the overtone spectrum of the complex, is to lengthen the terminal trans CH bond of the 1,3-butadiene ligand by 0.0024 Å while the terminal cis bond does not change significantly. The assignments of the terminal cis, terminal trans, and nonterminal CH stretch overtone absorptions are supported by ab initio geometry optimization calculations of the trans and cis butadiene and the BDIT complex. The spectrum of the complex possessed three additional bands not seen in the free ligand. These bands originate from vibrational coupling induced by the presence of the metal.