The mechanisms of iron-catalyzed [4 + 2] cycloadditions of unactivated dienes were investigated using density functional theory calculations. The calculation results show that the reaction involves sequential key steps of an initial ligand exchange followed by oxidative coupling, isomerization to form a seven-membered ferracycle intermediate, and C-C reductive elimination to form the cyclohexene product. The C-C reductive elimination step is shown to be the rate-determining step of the catalytic cycle. Moreover, energy profiles with three possible spin states (