In the present Communication, we simulate the full singlet-triplet-quintet dynamics of a Fe(II) polypyridine by full dimensional spin-vibronic trajectory surface hopping. Importantly, we herein achieve the so far highest complexity in terms of nuclear dimensionality and electronic states, including all the 3N -6 nuclear coordinates as well as all accessible spin states in the simulation. We select the [Fe(terpy) 2 ] 2+ complex (Figure , terpy = 2,2':6',2"-terpyridine), the principal molecule of our investigations across the years, as a suitable representant of the Fe(II) polypyridines. Crucially, we present clear evidence that the sub-ps conversion to the quintet state occurs via the sequential pathway, with branching through the two 3 MC components, 3 T 1g and 3 T 2g (which differ in whether the 3d x 2 -y 2 or 3d z 2 e * g orbital is singly occupied); note that for simplicity, we use here octahedral notations. In addition, in agreement with a recent combined X-ray scattering/spectroscopy experiment, we observe non-exponential dynamics and coherent oscillations along the Fe-N breathing mode.