Generation and characterization of a fairly stable triplet carbene

Most molecules are held together by covalent bonds-electron pairs jointly s hared by the two atoms that are linked by the bond. Free radicals, in contr ast, have at least one unpaired electron. In the case of carbon-based radic als, the carbon atom at the radical centre no longer makes four bonds with other atoms as it would do in its normal, tetravalent state. The presence o f unpaired electrons renders such radicals highly reactive, so they normall y occur only as transient intermediates during chemical reactions. But the discovery(1,2) by Gomberg in 1900 of triphenylmethyl, the first relatively stable free radical containing a central trivalent carbon atom, illustrated that radicals with suitable geometrical and electronic structures can be s table. Compounds containing a divalent carbon atom that uses only two of it s four valence electrons for bonding are usually less stable than Gomberg-t ype radicals with trivalent carbon(3-5). Although the role of these so-call ed carbenes in chemical reactions has long been postulated, they were unamb iguously identified only in the 1950s. More recently, stable carbenes have been prepared(6,7), but the singlet state of these molecules(6-12), with th e two nonbonding valence electrons paired, means that they are not radicals . Carbenes in the second possible electronic state, the triplet state, are radicals: the two nonbonding electrons have parallel spins and occupy diffe rent orbitals(13,14). Here we report the preparation and characterization o f a triplet carbene, whose half-life of 19 minutes at room temperature show s it to be significantly more stable than previously observed triplet carbe nes(15-17).