Examples of chemical reactions that occur in the solid state without disrupting the crystallinity of the original compound are rare, but Cathleen M. Crudden and coworkers of Queen’s University, in Kingston, Ontario, have discovered an unprecedented back-to-back example of such transformations (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201103316). The chemists were exploring why electron-rich rhodium(I) N-heterocyclic carbene complexes resist oxidation to form rhodium(III) complexes. In one reaction they found that treating a dirhodium ethylene complex with a bulky imidazolylidene ligand in an inert nitrogen atmosphere led to a rhodium dinitrogen complex. Subsequently exposing single crystals of this complex to oxygen triggered a color change from yellow to blue, but instead of oxygen oxidizing rhodium, it formed a stable rhodium(I) dioxygen complex. The same thing happened when the researchers subsequently exposed the crystals of the rhodium (I) dioxygen complex to carbon monoxide: The crystals changed color from blue to brown as CO displaced O2. When studying the X-ray crystal structures of the N2, O2, and CO complexes, Crudden’s team was surprised to find that the two nonreversible ligand-exchange reactions had occurred without loss of crystallinity and with only minor effects on molecular arrangement in the crystals. Because CO binds best to the rhodium complex, the researchers believe the complex could be the basis of a CO sensor.