Periodic burst-like photochemical bond splitting in metal carbonyls and cyclohexadiene

Recently, Schori et al., Nat. Commun., 2025, 16, 4767 observed by time-resolved X-ray diffraction of Fe(CO)₅, excited at 267 nm, a burst-like CO elimination, which was synchronized with a Fe-CO stretch vibration excited in the Franck-Condon region. Another recent work, using time-resolved electron diffraction (Ma et al., preprint, 2025, DOI: https://doi.org/10.21203/rs.3.rs-6125327/v1), provided supplementary results. The periodic bursts were predicted before (Banerjee et al., Nat. Commun., 2022, 13, 1337) by molecular dynamics simulations. They were interpreted by periodic passage of the wave packet from the initially excited bound state to a repulsive state through the crossing of the potentials. Whereas this mechanism is appealing, there are still open questions. Attention is drawn here on two similar cases, which seem more clear-cut: CO elimination from group-6 metal hexacarbonyls such as Cr(CO)₆ and the ring opening of 1,3-cyclohexadiene. The comparison not only shows that the phenomenon may be more general but also suggests some additional interpretations concerning conical intersections, slope directions, assignment of oscillations and mechanisms of energy redistribution.