Abstract
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(2)(II)(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(2)(II)(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.