DFT studies are reported of a monomeric iron dialkyl for which oxygen atom insertion into metal-methyl bonds occurs with O-2: FeMe2 + O-2 ? Fe(OMe)(2). Computation of the reaction coordinate implicates the intermediacy of Fe-III-peroxo, Fe-VI-dioxo, and Fe-IV-oxo intermediates, connected by O-2 oxidative addition and two methyl migration steps. Analysis of the reaction of O-2 with d(6)-Fe(Me)(2) indicates that oxy-insertion for this iron complex occurs with lower free energy barriers than competing homolytic/radical pathways, exploiting "spin-flip" processes via minimum energy crossing points (MECPs).