The Tebbe reagent has been renowned for decades for its utility as a methylene-transfer reagent. Although DuPont chemist Frederick N. Tebbe first isolated the titanocene-aluminum complex in 1974, it has eluded definitive structural characterization because of its high reactivity, chemists believe. Researchers at Indiana University led by Rick Thompson and Daniel J. Mindiola, both now at the University of Pennsylvania, have solved the structure of the Tebbe reagent cocrystallized with an inert impurity (Organometallics2013, DOI: 10.1021/om401108b). The project began when Thompson was a graduate teaching assistant for an advanced undergraduate inorganic chemistry lab in which the students had to prepare and use the Tebbe reagent. Thompson noticed indications of an impurity and pursued an analysis that culminated in the crystal structure, Mindiola says. The geometry of the reagent complex is largely as Tebbe predicted and studies of other compounds had indicated, with a (C₅H₅)₂Ti group bridged to an Al(CH₃)₂fragment through Cl and CH₂ groups. In the impurity, the bridging CH₂ is replaced by a second Cl. What surprised Mindiola and colleagues was the impurity, which makes up 40 to 50% of their crystallized reagent samples and had gone unrecognized in the past, even though Tebbe had predicted it.