The 3 MC and 5 MC populations exhibit oscillations with a ∼300 fs period (which corresponds to the breathing mode), as seen in Figure . We find that this is a consequence of population transfer between 3 T 1g , the lower 3 MC component, and the MC states (see Supplementary Figure ). Its origin can be identified by Figure , which displays the time evolution of the average Fe-N bond lengths (black and blue curves). We assign these oscillations to the coherent nuclear dynamics along the breathing motion, which is activated in the quintet state. In agreement with the time-resolved X-ray scattering experiment, we find that the structural Fe-N variations are slightly delayed with respect to the 1,3 MLCT → 3 MC electronic population dynamics. The reason for this is that only the MC states can launch the ballistic Fe-N expansion, which first have to be populated from the MLCT states. We interpret the large increase in Fe-N bond lengths in the MC states, as driving force of the vibrational coherence. Our coherent oscillations are consistent with various TR experiments, 5,7,9,24 however, in contrast to the experimental observations, we do not observe decoherence (damping). This is due to the absence of vibrational cooling (i.e., a solvent) in our theoretical model, which though clearly cannot affect the LS → HS (quintet) process that is several times faster. The lack of the inclusion of damping is causing the only relevant difference from experiments, as in the latter only the first hump of the population oscillation is seen due to decoherence. Finally, we calculated the transient X-ray scattering data from the simulated evolution of the molecular structure. In agreement with a series of ultrafast studies on TM complexes, we find that the coherent oscillations are directly observable by X-ray scattering, see the simulated difference signal displayed in yellow in Figure .