The appearance of two types of crystals—some red, some blue—in a reaction vessel thought to contain a single product might spell trouble; impurities, for example. But an observation of that type, made in a catalyst synthesis study at Pacific Northwest National Laboratory, has instead deepened understanding of isomerization in complexes with agostic bonds. An agostic bond is a type of bonding between a transition-metal atom (M) and a C–H unit that involves a three-center, two-electron arrangement (C–H–M). The bond stabilizes some organometallic complexes, including ones at the center of homogeneous catalytic processes. Researchers have known for years that some agostic complexes undergo rapid interconversion in solution between structurally isomeric forms, each of which retains the original agostic bond. In just a handful of cases, they have simultaneously identified the pair of solution-phase isomers. Now, PNNL’s Edwin F. van der Eide, Ping Yang, and R. Morris Bullock report that isomerization can unexpectedly cause an agostic molybdenum phosphine complex to crystallize in two forms (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201305032). Each isomer has a distinct molecular orbital configuration and hence a distinct color, they say.