The ultrafast nonadiabatic dynamics of a prototypical Cu(I)phenanthroline complex, Cu(dmp)(2) (dmp = 2,9-dimethyl-1,10-phenanthroline), initiated after photoexcitation into the optically bright metal-to-ligand charge-transfer (MLCT) state (S-3) is investigated using quantum nuclear dynamics. In agreement with recent experimental conclusions, we find that the system undergoes rapid (similar to 100 fs) internal conversion from S-3 into the S-2 and S-1 states at or near the FranckCondon (FC) geometry. This is preceded by a dynamic component with a time constant of similar to 400 fs, which corresponds to the flattening of the ligands associated with the pseudo JahnTeller distortion. Importantly, our simulations demonstrate that this latter aspect is in competition with subpicosecond intersystem crossing (ISC). The mechanism for ISC is shown to be a dynamic effect, in the sense that it arises from the system traversing the pseudo JahnTeller coordinate where the singlet and triplet states become degenerate, leading to efficient crossing. These first-principles quantum dynamics simulations, in conjunction with recent experiments, allow us to clearly resolve the mechanistic details of the ultrafast dynamics within Cu(dmp)(2), which have been disputed in the literature.