on the central carbon atom. Further calculations revealed the two highest-occupied molecular orbitals to be primarily localised on the central carbon atom as two lone pairs, like with the hexaphenylcarbodiphosphorane, albeit with slightly more delocalisation of the ?-symmetric orbital onto the N-heterocyclic carbene carbon atoms due to their improved ?-accepting properties. This is suggestive of a donor-acceptor interaction between the N-heterocyclic carbene ligands and a formally carbon (0) atom with two free lone pairs.

Other carbene structures