Stabilization of Linear C<sub>3</sub> by Two Donor Ligands: A Theoretical Study of L‐C<sub>3</sub>‐L (L=PPh<sub>3</sub>, NHC<sup>Me</sup>, cAAC<sup>Me</sup>)**
Sai Manoj N. V. T. Gorantla, Sudip Pan, Kartik Chandra Mondal, Gernot Frenking
Abstract
Abstract 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.