Enzymes containing heme, nonheme iron, or copper active sites play an essential role in the dioxygen binding and activation for substrate oxidation. The conceptual challenges to the quantitative modeling of this primary catalytic step arise from (1) instrinsic electronic nonadiabaticity of the spin flip events of the triplet dioxygen molecule (3O2), mediated by spin-orbit coupling and (2) possible heat dissipation channels, due to the high exothermicity of dioxygen binding processes. Herein, the spin-forbidden dioxygen binding dynamics of a reduced heme model was directly investigated in terms of the nonadiabatic trajectory surface-hopping dynamics, involving the coupled singlet, triplet and quintet states. This work reveals the complexity of this elemental reaction, and the binding/dissociation dynamics of iron peroxo species is important to interpret the subsequent H atom abstraction reaction step. Furthermore, we identify nonadiabatic dynamical effects that could not be observed through traditional calculations of static geometries.