Abstract: | Research on O2 activation at ligated CuI is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a CuII‐O‐CuII species, which is more special than it appears at first sight: a single oxo ligand between two CuII centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when CuII‐O‐CuII cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu‐grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features. |