NHC-Stabilised Acetylene-How Far Can the Analogy Be Pushed?

Experimental studies suggest that the compound (NHC ) C H can be considered as a complex of a distorted acetylene fragment which is stabilised by benzoannelated N-heterocyclic carbene ligands (NHC )→(C H )←(NHC ). A quantum chemical analysis of the electronic structures shows that the description with dative bonds is more favourable than with electron-sharing double bonds (NHC )=(C H )=(NHC ).