Theoretical studies on the strength and nature of the bond in Ni( ii ), Cu( ii ) and Zn( ii ) complexes of a large series of symmetric salen ligands

Herein, we describe a theoretical study on the strength and nature of metal-ligand bonds in Cu(II), Ni(II), and Zn(II) complexes of fifty substituted salen ligands. Initially, the geometries of all complexes were optimized at the M06L level of theory using the def2-TZVPP basis set. In continuation, the metal-ligand interaction energies were calculated and compared. NBO and energy decomposition analyses (EDA) calculations were used to investigate the effect of metal type, substituents and ligand charge on the nature and strength of metal-ligand bonds in these compounds. The results showed that Ni(II) and Cu(II) complexes have the largest metal-ligand interaction energy values and bond dissociation energies, respectively, among the metal complexes studied in this work. Furthermore, the positively charged complexes have the lowest interaction energy values, and a negatively charged complex has the largest interaction energy. Indeed, the bonding energy in one anionic salen complex can be more than twice that of a cationic one. The results also indicated that the strength of the metal-salen bond in neutral complexes changes by up to 6 percent by changing the substituents on the phenylene rings. The EDA calculations show that the metal-salen bonds in Zn(II) complexes, compared to Ni(II) and Cu(II) complexes, are more electrostatic in nature.