A theoretical study on the structure and nature of the M N bond in some potential pharmacologically active complexes; [(CO)4M BIIM(R)] {M = Cr, Mo and W, R = H, F, Cl and Br} with R(3,3 substitution), R(4,4 substitution), R(5,5 substitution) and R(3,3 4,4, 5,5 substitutions) was carried through density functional BP86 and ab initio MP2 methods using the def2-TZVPP basis set. The M N bond was analyzed using NBO, AIM, EDA and ETS-NOCV to obtain the electrical charge and energy decomposition analysis (EDA). The results showed that [(CO)4W BIIM(H)] and [(CO)4Cr BIIM(F)] are related to the D form complex with the biggest and the smallest DEint, respectively. Additionally, the M N bond interaction energies (DEint) increase in the group from top to bottom, from the lighter to the heavier elements (Cr < Mo < W). The EDA suggests that the [(CO)4M BIIM(R)] attractive interactions come mainly from DEelstat, which provides 60% of DEint, while 40% comes from DEorb. The applicability of the ETSNOCV scheme is demonstrated for the complexes and it is shown that the DEorb term arises mainly through [(CO)4M BIIM(R)] r-donation (in the range 54–61%) while a smaller part (27%) comes from [(CO)4M?BIIM(R)] p-back donation. The nature of the M N bond in the complexes was also studied by AIM and NBO analyses. The data has confirmed that the M N bond is partly covalent.
