A theoretical study on the structure and nature of the C?E bond (E = Si, Ge, Sn, Pb) in hexavalent sixmembered chelate rings of some bis-carbene complexes, [bis-NHC(R)?EBr2H2] (E = Si to Pb; R = H, F, Cl, Br, CH3, CF3) with R(2,2 substitution) and R(4,4 substitution) on the bis-NHC ligand, with both the B3LYP and BP86 methods using the def2-TZVPP basis set has been reported. The calculated interaction energies between the bis-NHC(R) and EBr2H2 fragments in the above complexes showed that among all the complexes with the same substituents, the [bis-NHC(R)?SiBr2H2] and [bis-NHC(R)?PbBr2H2] complexes have greatest and smallest amounts of interaction energy, respectively. In continuation, the amount of the natural charges of the elements in the complexes, the hybrid orbital types, WBI and molecular orbitals were calculated through NBO analysis. Also AIM and EDA analyses were used to detect the presence of bond critical points (BCPs) and the nature of the C?E bonds in the complexes. The results confirmed that the contribution of the electrostatic interactions in the C?E bonds of the present complexes is predominantly more than 50%.
