“…One of the key features of NHCs compared to other classes of carbenes is their stability. Simple carbenes, like the parent methylidene (“CH 2 ”, 1 , Chart ), decompose above cryogenic temperatures, cyclopropanate alkenes, , dimerize, − and insert into otherwise unreactive C–H bonds. ,, These reactions can be violently exothermic . In contrast, NHCs can be stored, bottled, and even distilled. , Although NHCs are far superior in stability to other types of carbenes, they still do not surpass the stability of classic phosphines such as PPh 3 .…”