Dynamic Processes on the IR Time Scale: Coalescence of CO Stretching Vibrational Bands in [(η4-Diene)Fe(CO)3] Complexes

Complex chemists should check the temperature dependence of the IR spectra of their compounds more often. The IR spectra of the Fe(CO)3 complexes with norbornadiene and 1,5‐cyclooctadiene at room temperature show only two CO bands, despite Cs symmetry, whereas at ca. 10 K the expected three bands are observed with about equal intensities. That is, a coalescence must take place by a very rapid intramolecular rearrangement whose activation energy is only ca. 2. 2 kcal mol−1.