The deactivation mechanism of photoexcited [Co(terpy)2]3+ (terpy = 2,2':6',2"-terpyridine) in acetonitrile is explored using transient absorption spectroscopy and density functional theory calculations. Based on the time-resolved experimental data and its theoretical assignment, we observe a quintet metal-centred (5MC) transient that decays with a time constant of 170 fs via 5MC # 3MC intersystem crossing (ISC). We report vibrational cooling within the 3MC state (characterized by a time constant of 5.7 ps) and a 3.4 ns 3MC excited-state lifetime. While both Fe(II) and Co(III) polypyridine complexes exhibit excited-state lifetimes on the order of a few nanoseconds, the governing excited-state dynamics are different. For Fe(II), the ns-lived excited state is the 5MC, and thus the lifetime is controlled by the weak second-order singlet-quintet coupling. On the other hand, for 3MC character for Co(III) means significantly stronger singlet-triplet coupling and the excited-state lifetime is controlled by unfavorable energetics governing triplet-singlet ISC (from 3MC to the singlet ground state).