The Flipping of CO Ligand Groups in Metal Carbonyl Compounds and its Frequency in Fe(CO)5

It has been proven qualitatively by a number of authors using variable temperature NMR experiments that most metal carbonyl complexes are nonrigid. A quantitative determination of the ligand exchange frequency veis often achieved by a line shape analysis or by measurement of the transverse relaxation timeT2using the Carr‐Purcell method. In the case of a “very fast” exchange, however, both methods prove unsuccessful. It is shown in this study that a simultaneous fit of IR or Raman spectra on the one hand and NMR spectra on the other can make possible the determination of vefor the “very fast” exchange and can also facilitate the determination of vein “slow” and “medium” exchange cases considerably. The ligand exchange frequency thus found for Fe(CO)5, 1.1 × 1010s−1, is unexpectedly high; comparison with variable temperature measurements on solid Fe(CO)5, yields similar energy barriers. A mechanism of exchange closely related to the “Berry mechanism” is proposed. Finally the consequences of this surprisingly large ligand exchange rate are discussed with respect to IR band assignments for molecular “fragments” M(CO)x(wherex=coordination number, and M is a transition metal, typically lanthanoid or actinoid).