Fig. 4. Structure parameters (the bite angle of OC–Fe–CO, and the bond lengths of Fe–X, Fe–C and C–X in Å) of the stationary points along the reaction coordinate for the oxidative insertion of Fe(CO)₄ into CH₄, CH₃Cl and C₂H₆, computed at ZORA-OPBE/TZ2P. Uppercase letters A, B, and C represent Fe(CO)₄ in the neutral singlet state, neutral triplet state and anionic (2−) singlet state, respectively. Numbers 1, 2, and 3 represent the three substrates CH₄, CH₃Cl, and C₂H₆, respectively. Lowercase letters a, b, and c represent the three stages along the oxidative reaction, namely the reactant complex, transition state and final product, respectively.