Effect of substituents at the heteroatom on the structure and ligating properties of carbodicarbenes and its silicon analogs: a theoretical study

Density functional calculations have been carried out to investigate the effect of substituents attached to the heteroatoms of N-heterocyclic carbenes (NHCs) on the structure and ligating properties of carbon(0) C(NHC)₂] and silicon(0) Si(NHC)₂] compounds. The substituents were found to have a profound role on the structure and ligating properties of these classes of compounds. Fluoro- and chloro-substituted carbon(0) compounds were found to have quasi-linear geometries in which their C(0) characteristics are “masked.” However, their C(0) characteristics become prominent in their protonated species. Large negative charges and shallow bending potential of the central C_c–C₀–C_c angle provide evidence for the “hidden C(0) characteristics” of these two compounds. Electron withdrawing substituents at N-atoms of the two NHCs dramatically decreases the basicity of these compounds. Both natural bonding and atoms in molecules analysis suggest that the most favorable Lewis structure of C(NHC)₂ and Si(NHC)₂ in their equilibrium geometries should be described (portrayed) as L=C=L and L → Si ← L, respectively, where L = NHCs.