Bond character and conformational equilibration of ethylene- and tetrafluoroethylenerhodium complexes from nuclear magnetic resonance spectra

The nuclear magnetic resonance spectra of π-cyclopentadienyldiethylenerhodium (l), π-cyclopentadienylethylene(su1fur dioxide)rhodium (2), and π-cyclopentadienylethylenetetrafluoroethylenerhodium (3) have been analyzed in terms of chemical shifts and coupling constants. There is relatively small difference between the proton-proton couplings, but a larger difference between fluorine-fluorine couplings of the coordinated and free alkenes, which suggests that coordination with rhodium produces a sizable change in hybridization of tetrafluoroethylene carbons. "π-Alkene complex" may be a misnomer for 3. The barrier to rotation of the ethylenes in 1 about the carbon-rhodium bonds has been reinvestigated and found to have an activation energy of 15.0 ± 0.2 kcal/mol instead of about 6 kcal as previously reported. The E, value for rotation of the ethylene in 2 is substantially less, 12.2 ± 0.8 kcal/mol, while that of 3 is somewhat more than for 2, 13.6 ± 0.6 kcal/mol. Rotation of coordinated tetrafluoroethylene does not appear to be detectable up to 100º, the point where 3 begins to decompose.