Although potentially powerful, molecular oxygen is an inert oxidant due to the triplet nature of its ground state. Therefore, many enzymesse various metal cations (M) to produce singlet active species M(n) O(2) . In this communication we investigate the topology of the Electron Localization Function (ELF) within five biomimetic complexes which are representative of the strategies followed by metalloenzymes to activate O(2) . Thanks to its coupling to the constrained DFT methods the ELF analysis reveals the tight connection between the spin state of the adduct and the spatial organization of the oxygen lone pairs. We suggest that enzymes could resort to spin state control to tune the regioselectivity of substrate oxidations.
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