Even the normal is abnormal: N-heterocyclic carbene C² binding to a phosphaalkene without breaking the P=C π-bond

The reaction of MesP=CPh2 with the least sterically demanding N-heterocyclic carbene (NHC = IMe) results in formation of the 'abnormal' (C4-substituted) 4-phosphino-NHC (1). In contrast, reaction with Me2IMe gives the unprecedented 'normal' C2 adduct, Me2IMe → P(Mes)=CPh2 (2). Particularly striking is the asymmetric and weak bonding of the NHC to the P=C moiety in 2. DFT calculations indicate that the P=C natural bond order in 2 (1.54) still reflects significant π-character to the bond (cf. MesP=CPh2: NBO = 1.98). Further computational analysis suggests that π-delocalization into the remote C-phenyl substituents is key to stabilizing the NHC adduct.