In this Communication, we revealed the branching mechanism of Fe(II) polypyridine complexes by full-dimensional simulation of the entire singlet-triplet-quintet dynamics of [Fe(terpy) 2 ] 2+ . We found that the quintet HS state is populated on the sub-ps timescale by two sequential pathways involving the two 3 MC components 3 T 1g and 3 T 2g . Importantly, we observe non-exponential dynamics and coherent oscillations, which emphasises the essence of explicit dynamics frameworks. These results are consistent with various experimental TR data, offer decision to a decade long debate, and demonstrate the power of our dynamics theoretical approach to complement and interpret ultrafast experiments.