Stabilization of Linear C(3) by Two Donor Ligands: A Theoretical Study of L‐C(3)‐L (L=PPh(3), NHC(Me), cAAC(Me))

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L‐C(3)‐L with L=PPh(3) (1), NHC(Me) (2, NHC=N‐heterocyclic carbene), and cAAC(Me) (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C(3) moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C(3) fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron‐sharing double bonds between neutral fragments (cAAC(Me))(2) and C(3) in the respective electronic quintet state yielding (cAAC(Me))=C(3)=(cAAC(Me)). In contrast, 1 and 2 possess electron‐sharing and dative bonds between positively charged ligands [(PPh(3))(2)](+) or [(NHC(Me))(2)](+) and negatively charged [C(3)](−) fragments in the respective doublet state.