## 3 Results and Discussion
### Antibacterial Activity  
Antibacterial activities of the ligands, their metal complexes and standard antibiotic drug (tetracycline) were performed against gram negative bacteria (_Escherichia coli_, _Serratia marcescens_ and _Pseudomonas aeruginosa_) and against gram positive bacteria (_Bacillus Subtilis_ and _Staphylococcus aureus_) using Muller Hinton agar medium by disk diffusion and broth dilution methods are shown in Figures 5 and 6, respectively. Figure 4: Proposed structure for **CoL\({}_{3}\)** and **CoL\({}_{6}\)** complexes. Figure 5: Graphical presentation of antibacterial activity of ligands against bacterial strains, measuring the inhibition zone (mm). DMSO: dimethyl sulfoxide. From Figure 5 it can be observed that the \(\mathbf{L_{3}}\) and \(\mathbf{L_{6}}\) with OH group had relatively higher antibacterial activity compared to the other ligands tested against the bacteria strains. These ligands had better inhibitory effect against _Pseudomonas aeruginosa_ with diameter inhibition zone of 15 and 14 mm, respectively. In contrast, the \(\mathbf{L_{4}}\) with one NO\({}_{2}\) group showed no antibacterial activity against tested bacteria strains. As can be seen from the antibacterial activity of metal complexes in Figure 6, among all these complexes the \(\mathbf{CoL_{3}}\) compound showed the higher antibacterial activity with inhibition zone of 14, 12, 12, 14 and 17 mm against _B. Subtilis_, _S. aureus_, _E. coli_, _S. marcesen_ and _P. aeruginosa_, respectively. The Minimal Inhibitory Concentration (MIC) values of all the synthesized compounds were also recorded and results are presented in Table 5. From the data listed in Table 5, the MIC values of metal complexes were lower than that of the parent ligands. The antibacterial activities of metal complexes (\(\mathbf{CoL_{1}}\) to \(\mathbf{CoL_{6}}\)) against _P. aeruginosa_ were more effective than the other tested bacteria strains with MIC value in the range of 0.62 to 2.5 mg/mL. In comparison these metal complexes were less effective against _E. coli_. It is observed from this study that metal chelates have a higher activity when compared to the parent ligands. Such increased activity of the metal chelates can be explained on the basis of [PERSON]'s concept and chelation theory [28; 29; 30; 31]. According to [PERSON]'s concept of cell permeability \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compounds** & _B. subtilis_ & _S. aureus_ & _E. coli_ & _S. marcesen_ & _P. aeruginosa_ \\ \hline \(\mathbf{L_{1}}\) & 2.5 & 10 & 5 & 5 & 2.5 \\ \(\mathbf{L_{2}}\) & 2.5 & 10 & 2.5 & 5 & 2.5 \\ \(\mathbf{L_{3}}\) & 1.25 & 5 & 5 & 2.5 & 0.31 \\ \(\mathbf{L_{4}}\) & - & - & - & - & - \\ \(\mathbf{L_{5}}\) & 2.5 & 1.25 & 2.5 & 5 & 2.5 \\ \(\mathbf{L_{6}}\) & 2.5 & 2.5 & 2.5 & 2.5 & 1.25 \\ \(\mathbf{CoL_{1}}\) & 1.25 & 2.5 & 2.5 & 2.5 & 2.5 \\ \(\mathbf{CoL_{2}}\) & 1.25 & 5 & 10 & 2.5 & 1.25 \\ \(\mathbf{CoL_{3}}\) & 0.15 & 2.5 & 1.25 & 0.31 & 0.15 \\ \(\mathbf{CoL_{4}}\) & 2.5 & 1.25 & 1.25 & 1.25 & 0.62 \\ \(\mathbf{CoL_{5}}\) & 1.25 & 1.25 & 1.25 & 0.62 & 0.62 \\ \(\mathbf{CoL_{6}}\) & 1.25 & 1.25 & 2.5 & 1.25 & 0.62 \\ \(\mathbf{Tetracycline}\) & 5 & 2.5 & 5 & 5 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Minimal inhibitory concentration (mg/mL) ligands and metal complexes based on broth dilution. method. Figure 6: Graphical presentation of antibacterial activity of Co complexes against bacterial strains, measuring the inhibition zone (mm). the lipid membrane that surrounds the cell favors the passage of only lipid soluble materials due to which liposolubility is an important factor that controls antimicrobial activity. On chelation, the polarity of the metal ion is reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups. Further, it increases the delocalization of p-electrons over the whole chelate ring and enhances the lipophilicity of the complex.