**3.1 Denticity of Schiff base ligands**

Schiff bases are classified into bidentate, tridentate, tetradentate, and polydentate ligands, which can form extremely stable complexes with transition metal ions (**Figure 5**). Assume they have different functional groups such as -OH, -NH2, or -SH; the resulting Schiff bases can act as mixed-donor ligands in bi-, tri-, tetra-, and higher

#### **Figure 5.**

*Classification of Schiff bases into bidentate, tridentate, tetradentate, and polydentate ligands.*

*Schiff Bases and Their Metal Complexes: Synthesis, Structural Characteristics and Applications DOI: http://dx.doi.org/10.5772/intechopen.108396*

coordination modes [48–51]. Multivalent Schiff base ligands easily form complexes with bidentate, tridentate, and tetra- or polydentate metal ions at different oxidation states. Donor atoms (N, O, S) can be found in bidentate ligand (NN or ON), tridentate ligands (NNN, ONO, NNS or ONS), and tetradentate ligands (ONNO, NNNN, NSNO).

#### **3.2 Symmetrical and asymmetrical Schiff bases**

The Schiff bases' great affinity for chelation towards inner and non-inner transition metal ions is employed to produce stable complexes. Unsymmetrical ligands do not have a rotation or mirror axis of symmetry and bind to a metal ion with two groups, and symmetrical ligands have [52, 53]. It has been argued that unsymmetrical Schiff base ligands are superior to their symmetrical counterparts due to their ability to more accurately predict the geometry of metal ion binding sites in biological systems that contain metal ions and their ability to more easily combine natural and synthetic structural components (**Figure 6**).

### **3.3 Homoleptic and heteroleptic Schiff base metal complexes**

The primary distinction between homoleptic [54] and heteroleptic [55] complexes is that homoleptic complexes have identical ligands linked to a metal centre.

#### **Figure 6.**

*Symmetrical and asymmetrical salphen Schiff bases.*

#### **Figure 7.**

*Homoleptic and heteroleptic Schiff base metal complexes.*

*Schiff Bases and Their Metal Complexes: Synthesis, Structural Characteristics and Applications DOI: http://dx.doi.org/10.5772/intechopen.108396*

#### **Figure 9.**

*Achiral, chiral, trans and cis Schiff base metal complexes [62–65].*

In contrast, heteroleptic complexes have at least one distinct ligand coupled to the complex's metal centre (**Figure 7**).

#### **3.4 Mononuclear and polynuclear Schiff base metal complexes**

A single metal atom or ion is contained within the most basic type of Schiff base metal complex, and it is surrounded by monodentate, bidentate, tridentate, and polydentate ligands. Polynuclear Schiff base metal complexes are attributed to the presence of two or more atoms of metal, or ions, co-ordinated within a single coordination sphere. The two atoms may be linked together by direct metal-metal bonds, bridging ligands, or all of these things. As versatile ligands, Schiff bases form various polynuclear metal complexes such as homonuclear and heteronuclear. These flexible ligands have the ability to act as monodentate, bidentate, or polydentate, and they can be engineered to produce mononuclear, dinuclear, or polynuclear metal-organic frameworks. It is possible to change the nuclearity of Schiff base complexes; for example, it is possible to synthesize either mono- or dinuclear complexes using nearly identical ligands and synthetic processes for both types of complexes (**Figure 8**).

#### **3.5 Achiral and chiral Schiff base metal complexes**

A chiral Schiff base metal complex is not superimposable with its own mirror image because the two structures are not identical in all respects. The mirror image of an achiral Schiff base metal complex is identical to the complex itself and can be superimposed on it. The phenomena of optical activity have traditionally been defined in terms of asymmetry and dissymmetry; however, the term chirality has recently superseded these earlier classifications. Chiral entities exist as two species with the same chemical constitution. The only way they are distinguishable from one another is that they have the opposite configuration of an object and the mirror image of that thing. Chemical compounds can be said to be stereoisomers if their chemical constitutions are same but their spatial arrangements of their atoms are different. Chiral refers to the property of molecules that prevents them from being brought into coincidence with their mirror copies by the use of stiff motions (**Figure 9**).
