The13C NMR spectra for members of the series Fe(CO)5minus;x(PF3)x, wherexequals; 0, 1, 2, 3, and 4 were studied for temperatures between plus; 25deg; and minus; 100 deg;C and carbonhyphen;phosphorus and carbonhyphen;fluorine spin coupling constants evaluated. The infrared spectra were also studied within the same temperature limits in the COhyphen;stretching region and mole fractions for the different isomers within each compositionxwere evaluated at each temperature. The magnitude of the observedJCPspinhyphen;coupling constants can be accounted for by using the experimentally determined mole fractions and thetrans, cis, andvicinalcoupling 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 timehyphen;averaged13C chemical shifts, the chemical shift difference dgr;sgr;0between the axial and equatorial sites is found to be 17.7plusmn;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 1.1times;1010secminus;1in the fivehyphen;coordinate nonrigid Fe(CO)5molecule. This is the first time such a surprisingly fast flip frequency has been measured for such a large molecule.
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