Stabilization of Linear C₃ by Two Donor Ligands: A Theoretical Study of L-C₃-L (L=PPh₃, 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), 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 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 (cAAC^Me)₂ and C₃ in the respective electronic quintet state yielding (cAAC^Me)=C₃=(cAAC^Me). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh₃)₂]⁺ or [(NHC^Me)₂]⁺ and negatively charged [C₃]⁻ fragments in the respective doublet state.