Intersystem crossing (ISC) rates of transition-metal complexes are determined by the complex interplay of a molecule's electronic and structural dynamics. To broaden our understanding of these key factors, we investigate the case of the prototypical d(8)-d(8) dimetal complex [Pt(ppy)(mu-(t)Bu(2)pz)](2) using broad-band transient absorption anisotropy in combination with ultrafast fluorescence up-conversion and ab initio calculations. We find that, upon excitation of the molecule's metal-metal-to-ligand chargetransfer transition, ISC occurs in hundreds of femtoseconds from the lowest excited singlet state S-1 to the triplet state T-2, from where the energy relaxes to the lowest energy triplet state T-1. ISC to the T-2 state, rather than T-1, is further rationalized through supporting arguments. Observed vibrational coherences along the Pt-Pt mode are attributed to the formation of nuclear wavepackets on the ground and excited electronic states that dephase prior to ISC because of the structural flexibility of the complex. Beyond demonstrating the relationship between the energy relaxation and structural dynamics of [Pt(ppy) (mu-(t)Bu(2)pz)](2), our results provide new insights into the photoinduced dynamics of d(8)-d(8) dimetal complexes more generally.