Infrared Dynamics of Iron Carbonyl Diene Complexes

The temperature dependence of the low-frequency C–O bands in the IR spectrum of [(η⁴-norbornadiene)­Fe­(CO)₃], reminiscent of signal coalescence in dynamic NMR, was interpreted by Grevels (in 1987) as chemical exchange due to very fast rotation of the diene group. Since then, there has been both support and objection to this interpretation. We discuss these various claims involving both one- and two-dimensional IR and, largely on the basis of new density functional theory calculations, furnish support for Grevels’ original interpretation.