Geometrical isomerism in divalent-carbon intermediates

Three classes of monosubstituted carbenes (aryl-, vinyl-, carbonyl-) and one class of disubstituted carbenes (dicarbonyl-), all of them ground state triplets, can exist in more than one rotameric form with distinct zero field splitting parameters. Representative examples of these carbenes are discussed and selected aspects of triplet carbenes are evaluated, including their π spin density distribution, intramolecular H-abstraction, and cis-trans isomerization. Lastly, the structures assigned to two triplets generated by photolysis of dibenzoyldiazomethane are re-evaluated.