The temperature-dependentl3C NMR spectra for 12 compounds of the form (CO)4CoEX3(where E=C, Si, Ge, Sn, or Pb and X=F, Cl, CH3, C4H9, CH2C6H5, or C6H5) in CFCl2H solution have been recorded. At higher temperatures a single 13C carbonyl resonance is observed for all molecules, indicating a time averaging of the axial and radial carbonyl signals due to an intramolecular exchange process. At lower temperatures the exchange process is observed to slow; coalescence temperatures range from less than -160 °C when EX3=SnCl3to -10°C when EX3=CF3. Complete line shape fitting of the spectra permits determination of the activation parameters for the exchange for most of the compounds studied. The results indicate that the free energy barriers to intramolecular rearrangement, which range from about 4.5 to 11.8 kcal/mol, are determined mainly by the steric requirements of the EX3 group. The barrier increases with increasing steric requirement of the EX3 group, as measured by the van der Waals contact angle with adjacent CO groups. Enthalpies of activation are in the range 6-7 kcal/mol for trihalogeno groups, 8.3-9.3 kcal/mol for E(C6H5)3 groups, and 7-8 kcal/mol for trialkyltin groups. The experimental observations are discussed in the light of possible mechanisms for the intramolecular exchange.
ASJC Scopus subject areas
- Colloid and Surface Chemistry