Stabilization of Linear C<inf>3</inf> by Two Donor Ligands: A Theoretical Study of L-C<inf>3</inf>-L (L=PPh<inf>3</inf>, NHC^Me, cAAC^Me)**

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3-L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (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 C3 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 C3 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 (cAACMe)2 and C3 in the respective electronic quintet state yielding (cAACMe)=C3=(cAACMe). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3)2]+ or [(NHCMe)2]+ and negatively charged [C3]− fragments in the respective doublet state.