Bonding situation in silicon complexes [(L)₂(Si₂)] and [(L)₂(Si)] with NHC and cAAC ligands

Abstract The equilibrium geometries of the silicon carbene complexes [(cAAC)₂(Si₂)], [(NHC)₂(Si₂)], [(cAAC)₂(Si)], [(NHC)₂Si] and [(NHC-CH₂-NHC)(Si)] have been calculated with gradient-corrected density functional theory (DFT) at the BP86/def2-TZVPP level of theory. A detailed bonding analysis of the five molecules was carried out with the NBO method and with the help of the EDA-NOCV method which provides a detailed and quantitative insight into the nature of the interactions between Si_n and the carbene ligands. The silicon atom in L₂Si is more strongly bonded than diatomic Si₂ in L₂Si₂. The cAAC ligands are stronger donors and stronger acceptors than NHC. The relative contribution of the cAAC ← Si_n → cAAC π-backdonation to the total orbital interactions is higher than the NHC ← Si_n → NHC π-backdonation which agrees with the general assessment that cAAC is a stronger π-acceptor than NHC. Silicon-carbene bonding in [(NHC-CH₂-NHC)(Si)] is stronger than in [(NHC)₂Si] mainly because of stronger π-backdonation but weaker than in [(cAAC)₂(Si)]. The calculated charge distribution of the NBO method agrees with the EDA-NOCV data.