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^II₂(μ‐η²:η²‐O₂)]²⁺ core relative to its isomer with a cleaved O?O bond having a [Cu^III₂(μ‐O)₂]²⁺ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu^II₂(μ‐η²:η²‐O₂)]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‐η²:η²‐O₂ species and highlight the potential of this method for time‐resolved studies of reaction mechanisms.