Identifying the molecular structure of the active phase of a solid catalyst—and the changes such catalysts undergo during reactions—is notoriously challenging. Yet that’s exactly what University of California, Davis, chemical engineers Alper Uzun and Bruce C. Gates have done in a study of iridium complexes that catalyze ethene hydrogenation (J. Am. Chem. Soc., DOI: 10.1021/ja906553n). A detailed description of a catalyst’s molecular-scale evolution during reactions is critical to deciphering and customizing its activity and product selectivity. By using IR spectroscopy and a time-resolved X-ray absorption method, the team found that tetrairidium clusters inside zeolite Y cages reversibly convert to mononuclear Ir complexes in response to changes in the ethene/hydrogen ratio. In an ethene-rich atmosphere, tetrairidium clusters break up into the mononuclear form. Switching to a hydrogen-rich gas feed causes the “tetra” clusters to re-form, the team reports.