text: In 1970, Wanzlick's group generated imidazol-2-ylidene carbenes by the deprotonation of an imidazolium salt. Wanzlick as well as Roald Hoffmann, proposed that these imidazole-based carbenes should be more stable than their 4,5-dihydro analogues, due to Hückel-type aromaticity. Wanzlick did not however isolate imidazol-2-ylidenes, but instead their coordination compounds with mercury and isothiocyanate:
  In 1988, Guy Bertrand and others isolated a phosphinocarbene. These species can be represented as either a λ-phosphinocarbene or λ-phosphaacetylene:
  These compounds were called "push-pull carbenes" in reference to the contrasting electron affinities of the phosphorus and silicon atoms. They exhibit both carbenic and alkynic reactivity. An X-ray structure of this molecule has not been obtained and at the time of publication some doubt remained as to their exact carbenic nature.
  In 1991, Arduengo and coworkers crystallized a diaminocarbene by deprotonation of an imidazolium cation:
  This carbene, the forerunner of a large family of carbenes with the imidazol-2-ylidene core, is indefinitely stable at room temperature in the absence of oxygen and moisture. It melts at 240–241 °C without decomposition. The C NMR spectrum shows a signal at 211 ppm for the carbenic atom. The X-ray structure revealed longer N–C bond lengths in the ring of the carbene than in the parent imidazolium compound, indicating that there was very little double bond character to these bonds.
  The first air-stable ylidic carbene, a chlorinated member of the imidazol-2-ylidene family, was obtained in 1997.
  In 2000, Bertrand obtained additional carbenes of the phosphanyl type, including (phosphanyl)(trifluoromethyl)carbene, stable in solution at -30 °C and a moderately stable (amino)(aryl)carbene with only one heteroatom adjacent to the carbenic atom.