Theoretical studies including structural investigations, interaction and formation energy and nature of in C → M bonds in some adducts of “1+1” versus “1+2” nano-sized complexes of N-heterocyclic carbene (NHC(R)((R= H, F, Cl, Br, CH3) with heavy elements of group II (Ca, Sr, Ba) metallocenes using the M06 level and def2-SVP and def2-TZVP basis sets have been reported. The results in good agreementwith the experimental data showed that Ca complexes of the “1+1” and Sr and Ba complexes of “1 + 2” in both benzene and toluene as solvents, have been formed respectively. Also to evaluate the nature of the C → M bond in the compounds, AIM, NBO, EDA as well as ETS-NOCV to obtain the electrical charge and the energy decomposition analysis (EDA) was performed. The above results confirm that the electrostatic character of C → M bond of complexes studied here are larger than covalent character, and its contribution is N50%. The applicability of the ETS-NOCV scheme has also been demonstrated and it's shown that the ΔEorb term for both “1 + 1” and “1 + 2” nano-sized complexes mostly arises from C → M σ-donation.
