text: The two families above can be seen as special cases of a broader class of compounds which have a carbenic atom bridging two nitrogen atoms. A range of such diaminocarbenes have been prepared principally by Roger Alder's research group. In some of these compounds, the N–C–N unit is a member of a five- or six-membered non-aromatic ring, including a bicyclic example. In other examples, the adjacent nitrogens are connected only through the carbenic atom, and may or may not be part of separate rings.
  Unlike the aromatic imidazol-2-ylidenes or triazol-5-ylidenes, these carbenes appear not to be thermodynamically stable, as shown by the dimerisation of some unhindered cyclic and acyclic examples. Studies suggest that these carbenes dimerise via acid catalysed dimerisation (as in the Wanzlick equilibrium).
  Diaminocarbenes have diagnostic C NMR chemical shift values between 230 and 270 ppm for the carbenic atom. The X-ray structure of dihydroimidazole-2-ylidene shows a N–C–N bond angle of about 106°, whilst the angle of the acyclic carbene is 121°, both greater than those seen for imidazol-2-ylidenes.