Microwave Measurements of the Rotational Spectrum and Structure of (Butadiene)iron Tricarbonyl

The rotational spectra of 10 isotopomers of (butadiene)iron tricarbonyl were measured using a Flygare-Balle-type microwave spectrometer operating in the 5–17-GHz frequency range. The 30 measured rotational constants were used to determine 12 gas-phase structural parameters describing the cis-butadiene conformation and the locations and orientations of the butadiene and CO ligands relative to the iron atom. Kraitchman analyses of these microwave measurements, with single and multiple deuterium substitution, provide accurate data on the hydrogen atom coordinates in the complex. The terminal CH₂ groups of butadiene are found to be rotated by 28° out of the butadiene plane, and the CH₂ plane is folded away from the C₁-C₂ axis by 27° in a direction away from the iron atom. Least-squares fits to all rotational constants to determine structural parameters were also carried out, and the results are compared with coordinates from the Kraitchman method. Other structural parameters are compared with X-ray diffraction results on similar compounds. The four possible single-substitution ¹³C isotopomers were observed in natural abundance samples. Samples for the two D₁, two D₃, and D₄ substitutions on terminal H atoms were synthesized. Proton NMR spectra were analyzed to obtain improved chemical shifts and coupling constants for this complex.