Carbene radicals are a special class of organometallic carbenes. The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex (in particular low-spin cobalt(II) complexes) using diazo compounds and related carbene precursors.Cobalt(III)-carbene radicals have found catalytic applications in cyclopropanation reactions, as well as in a variety of other catalytic radical-type ring-closing reactions.