## 3 Results and Discussion
### Calculated Structures  
#### 3.9.1 Geometry of the Complexes  
We used the obtained results from the spectroscopic analysis (IR, NMR, UV-VIS, and EPR) we proceeded with a computational study to optimize the complex's geometry by the DFT approach. The optimized geometries optimized in the gas-phase of the ligand and its metal (II) complexes were presented with the number and labels of the atoms in Figure 7.  
Figure 5: Horowitz–Metzger curves of Mn(II) in different stages. Figure 6: Suggested structure for metal complexes. A selected geometric parameter around the coordination sphere such as bond length and bond angles for the free ligand and its metal complexes were extracted and tabulated it in Table 1.  
The bond length of the free Schiff base showed different characters after bind to the metal ions. The bond length C20-N22 became longer than the free ligand after complexation, whether O18-C4 became shorter by 0.04 and 0.06 A, respectively. There was a slight change in the C14-N22 and C14-N23 bond lengths around 0.01 or 0.02 A. In the metal complexes, the primary ligand is coordinated in a bidentate to all metal except Co(II) and Zn(II) that bonded via tridentate from the organic ligand. The new bonds with the Mn(II) ion, M-O18, and M-N22, were in the reported range of the ionic character of these bonds [39]. Moreover, the other new bonds between the Mn(II) and HO2 or Cl ion are presented in a covalent bond. Interestingly, all the bonds around Cu(II) ion, M-O, and M-N were ionic bonds with an average length of 1.97 A. In contrast, the bidentate Ni(II) complexes and tridentate Co(II) and Zn(II) compounds have covalent bonds or medium ionic atmosphere with bond length average = 2 A. The importance of bond angles in the coordination complexes comes from the ability to describe the geometric structure of the prepared compounds. Generally, the selected angles around the coordination site in the free ligand, C3-C4-O18 and C3-C20-N22, became bigger after binding to metal ions, while the bond angle between N22-C14-N23 has decreased. The value of the bond angles O18-M-N22, N22-M-N23, H2O-M-N22, and H2O-M-Cl in the Mn(II) and Ni(II) complexes (Table 1) indicate this complex adopts a distorted octahedral geometry around its respective central metal ion. The deviation from linearity of the bond angles H2O-M-O18, H2O-M-N23, O18-M-N23, H2O-M-N22, and H2O-M-Cl confirm this distortion. Similarly, the complexes of Zn(II) and Co(II) showed a distorted octahedral structure. In the Cu(II) complex, the values of the bond angle O18  
Figure 7: Optimized geometry, numbering system, and label for the Schiff base ligand and its complexes using B3 LYP/LAND. M-O41 and N22-M-N44 were 146.07\({}^{\circ}\) and 161.30\({}^{\circ}\) suggests a highly distorted tetrahedral geometry around the central Cu(II) ions. 9.2 Ground-State Properties and Global Reactivity Descriptors of the Schiff Base Ligand and Metal Complexes  
The frontier orbital energies are important theoretical parameters that contest the highest occupied and the lowest unoccupied molecular orbital energies (HOMO and LUMO, respectively) and the difference between them is called the energy gap (E\({}_{\text{HOMO}}-E_{\text{LUMO}}=\) E\({}_{\text{gap}}\)) [17]. The calculated energy values for the ligand and its metal complexes were extracted and tabulated with the quantum parameters in Table 2. Figure 8 presents the HOMO and LUMO electron densities calculated by the B3 LYP method using the 6-31G basis set for Schiff base and LAND for the metal complexes. For the free ligand, E\({}_{\text{gap}}\) was 4.09 eV decreased after complexing with metal ions. The small E\({}_{\text{g}}\) indicates the softtness and reactivity of the complexes. We can arrange the complexes in the following order, Cu(II) \(>\) Mn(II) \(>\) Co(II) \(>\) Ni(II) \(>\) Zn(II). Therefore, the Cu(II) complex presents good reactivity and reveals good toxicity and inhibition activity [40].