In this work, we used ultrafast two-dimensional infrared (2D IR) spectroscopy to follow how the polaritons and dark modes evolve in both pseudorotation and intramolecular energy redistribution (IVR) of Fe(CO)5 . We did so in a state-resolved manner, thereby meeting the desired criteria listed above. Fe(CO)5 features two infrared (IR) active vibrational bands, a doubly degenerate e ' mode at 1999 cm -1 involving three equatorial CO groups and an a2 " mode at 2022 cm -1 involving the axial CO groups (Fig. ). Harris and co-workers applied 2D IR spectroscopy and showed that Fe(CO)5 can rearrange from its D3h equilibrium geometry to a C4v transition state and back to D3h, during which the equatorial and axial CO ligands interconvert, leading to vibrational energy exchange between a2 " and e' modes . This process, referred to as Berry's pseudorotation, is a single barrier crossing and thus represents the essence of elementary reactions, although the product is indistinguishable from the reactant . Given that Berry's pseudorotation competes with IVR between a2" and e' modes whose transition dipoles are perpendicular to each other (Fig. top), Fe(CO)5 is an ideal testbed to understand how VSC affects single barrier crossing events and the branching ratio between various dynamical processes.