To quote from Wikipedia: in chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons . The most ubiquitous type of carbene of recent times is the one shown below as 1 , often referred to as a resonance stabilised or persistent carbene . This type is of interest because of its ability to act as a ligand to an astonishingly wide variety of

As I have noted elsewhere, Gilbert N. Lewis wrote a famous paper entitled “ the atom and the molecule “, the centenary of which is coming up.[cite]10.1021/ja02261a002[/cite] In a short and rarely commented upon remark, he speculates about the shared electron pair structure of acetylene,  R-X≡X-R (R=H, X=C). It could, he suggests, take up three forms. H-C:::C-H and two more which I show as he drew them.