Structural, spectroscopic, and chemical bonding analysis of Zn(II) complex [Zn(sal)](H₂O): Combined experimental and theoretical (NBO, QTAIM, and ELF) investigation

The Zn(II) complex of salen-like scaffold [Zn(sal)](H₂O) was synthesized and characterized by elemental analysis, IR, UV–Vis, and¹H-NMR spectroscopic techniques. The structure of complex was confirmed by single crystal X-ray diffraction studies. In the complex, Zn (II) was placed in the inner N₂O₂ compartment of the salen scaffold in square planar geometry and crystallized in the monoclinic space group P2₁/n. DFT and TDDFT calculations were performed to reproduce the experimentally observed structural and spectroscopic (IR and UV–vis) findings. The bonding of the Zn(II) framework in the [Zn(sal)](H₂O) complex was explored in depth. The theoretical approaches employed were perturbation theory within the context of the natural bond orbital (NBO) framework, and quantum theory of atoms in molecule (QTAIM) and electron localization function (ELF) analysis. The study begins by delineating the difference between the NBO and QTAIM approaches. This paper thus exhibits the supportive nature of NBO theory and QTAIM in discussion of the bonding in the [Zn(sal)](H₂O) complex, when both the methodologies are used in combination.