Valence‐to‐Core X‐ray Emission Spectroscopy as a Probe of O−O Bond Activation in Cu₂O₂ Complexes

Valence‐to‐Core (VtC) X‐ray emission spectroscopy (XES) was used to directly detect the presence of an O−O bond in a complex comprising the [Cu II 2 (μ‐η 2 :η 2 ‐O 2 )] 2+ core relative to its isomer with a cleaved O−O bond having a [Cu III 2 (μ‐O) 2 ] 2+ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu II 2 (μ‐η 2 :η 2 ‐O 2 )] 2+ core corresponds to the copper stabilized in‐plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O−O bond activation in μ‐η 2 :η 2 ‐O 2 species and highlight the potential of this method for time‐resolved studies of reaction mechanisms.