A tin analog of acetylene, a rare compound with a Sn≡Sn bond, is proving to have unusual reactivity: It reversibly adds two ethylene molecules across the triple bond under ambient conditions, a reaction normally forbidden to its carbon analog, report Yang Peng, Philip P. Power, and coworkers of the University of California, Davis (Science 2009, 325, 1668). Cycloaddition reactions involving alkenes or alkynes are classic transformations in organic synthesis, leading to stable products. These reactions are normally "symmetry forbidden," meaning that under standard conditions the molecular orbital overlap is insufficient for bonding to occur. This barrier can be overcome by boosting the energy of the system with heat or light. In contrast, alkene and alkyne analogs of silicon, germanium, tin, and lead can undergo cycloadditions under mild conditions. But until now, even those reactions were irreversible. The UC Davis team showed that once ethylene adds to the triple bond of the distannyne RSn≡SnR, where R is a bulky phenyl group, the ethylene molecules of the isolable adduct that forms can pop off with a gentle change in temperature or pressure.