## 6 Metal Complexes Based on Terpene Ligands and Their Biological Activities
### Antimicrobial Activity of Metal Complexes Based on Tepene Ligands  
A metal complex, as opposed to freely solvated metal ions or metal nanoparticles, is a well-defined arrangement of ligands centered on one or more metal centers. These compounds are distinguished by the fact that their characteristics can be modified in a manner similar to that used in conventional medication development [226]. Metals have a wide range of properties and almost infinite combinations of ligands to form complexes, with the number of coordinations ranging from 1 to 20 [227; 228], resulting in a rich and three-dimensional variety of chiral metal complex structures. In comparison, the geometric diversity of organic compounds is lower because carbon normally forms no more than four bonds. In biology, chirality is particularly significant, since it can change the characteristics and therapeutic actions of molecules. When a chiral chemical is used in a compound of medical treatments or antimicrobial agents, one of the enantiomers may be effective on the organism while the other is ineffective. This potential enables us to create substances with three-dimensional structures, as the use of chiral centers correlates with higher target selectivity and lower off-target effects [221; 229; 230; 231]. Metal ions, as well as terpen and their derivatives, have well-known antimicrobial properties. However, terpen and their derivatives are insoluble in water, necessitating the use of organic solvents such as ethanol, chloroform, diethyl ether, and DMSO to examine their activity against living organisms [232]. There is also the issue of volatility, despite the fact that monoterrenes are relatively stable. Sesquiterpenes and oily diterpenes, on the other hand, are less stable because they have more oxygen functional groups, making them biodegradable [233]. Biosourced terpenes might be used to synthesize complexes with antibacterial and antibofilm activities, such as the [(\(p\)-cymene)] [aminooxime L3]+Cl\({}^{-}\) (RuL3) complex based on (\(R\))-limonene, which is miscible in water; therefore, complexation overcomes the problem of solubility while also making it more stable. We previously demonstrated that the minimum inhibitory concentration (MIC) values for limonene were12.5 mg/mL when tested against _Escherichia coli_, _Staphylococcus aureus_, _Listeria monocytogenes_, _and Enterococcus faecalis_. In comparison, the MIC of the RuL3 complex (0.4 mg/mL) was approximately 30 times lower. Thus, limonene complexation with ruthenium increased its antibacterial effectiveness and appears to be a promising way to decreasing the amount of antimicrobial used against bacteria and biofilms [21]. Although there are many metal complexes based on terpene ligands, little is known about their biological activity [234; 235; 236]. Nonetheless, based on existing evidence, transition metal complexes with terpene ligands or Schiff base ligands are effective antimicrobial, anticancer, antifungal, and antioxidant agents (Table 5). Complexation protects against environmental variables, improves stability, avoids terpene component volatilization, and enhances antibacterial activity [237; 238; 239; 240].