Although nonheme iron(III)–iodosylarene complexes present amazing oxidative efficiency and selectivity, the nature of such complexes and related oxidation mechanism are still unsolved after decades of experimental efforts. Density functional calculations were employed to explore the structure–reactivity relationship of the iron(III)–iodosylbenzene complex, [Fe^III(tpena^–) (PhIO)]²⁺ (1), in thioanisole sulfoxidation. Our theoretical work revealed that complex 1 can evolve into two resonance valence-bond electronic structures (a high-valent iron–oxo species and a monomeric PhIO species) in thioanisole sulfoxidation to present different reaction mechanisms (the novel bond-cleavage coupled electron transfer mechanism or the direct oxygen-atom transfer mechanism) as a response to different substrate attack orientations.