By adding a bipyridine or a phenanthroline ligand to molybdenum metathesis catalysts, chemists in Germany have transformed the sensitive reagents into air-stable precatalysts that can be stored in an open flask for weeks (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201102012). Johannes Heppekausen and Alois Fürstner of the Max Planck Institute for Coal Research note that tetracoordinate molybdenum alkylidenes are “amongst the most powerful alkene metathesis catalysts known to date.” But because of their sensitivity to air and moisture, the compounds can be tough to work with. Heppekausen and Fürstner discovered that they could transform molybdenum alkylidenes into stable solid precatalysts by reacting them with either 2,2'-bipyridine or 1,10-phenanthroline in toluene at room temperature. To liberate the active catalyst, the chemists simply mix the precatalyst with zinc chloride in toluene at 100 °C. The resulting bipyridine•ZnCl2 or phenanthroline•ZnCl2 precipitate has no effect on the catalyst and need not be filtered. “Our new method combines the convenience of handling of a crystalline and bench-stable precatalyst with the benefits of a well-defined active species of proven versatility,” the researchers write.