Temperature dependent 13C NMR and ir studies in Fe(CO)5−x(PF3)x, X = 0, 1, 2, 3, and 4

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Abstract
The 13C NMR spectra for members of the series Fe(CO)5−x(PF3)x, where x = 0, 1, 2, 3, and 4 were studied for temperatures between + 25° and − 100 °C and carbon‐phosphorus and carbon‐fluorine spin coupling constants evaluated. The infrared spectra were also studied within the same temperature limits in the CO‐stretching region and mole fractions for the different isomers within each composition x were evaluated at each temperature. The magnitude of the observed JCP spin‐coupling constants can be accounted for by using the experimentally determined mole fractions and the trans, cis, and vicinal coupling constant increments. The magnitude of these three coupling constant increments were determined by using a set of observed coupling constants and the mole fractions. By using the mole fractions together with the time‐averaged 13C chemical shifts, the chemical shift difference δσ0 between the axial and equatorial sites is found to be 17.7±1.5 ppm. Using this value and relaxation rates determined in earlier work, the frequency of exchange between axial and equatorial sites is calculated to be 1.1×1010 sec−1 in the five‐coordinate nonrigid Fe(CO)5 molecule. This is the first time such a surprisingly fast flip frequency has been measured for such a large molecule.