**7. Schiff base metal complexes**

Schiff bases are widely used as ligands in coordination chemistry. Ligands are called Lewis bases because they donate electron pairs to the central atom. Since the nitrogen atom in the imine bond contains unpaired electrons, Schiff bases are electron donors, basic in character, and can form complexes with almost all transition metals. This atom, also known as azomethine nitrogen, is the primary bonding point for the Schiff base. In addition, the azomethine system, where the nitrogen atom is bonded with a double bond, can be a coordination site for d-metal ions suitable for back bonding, by means of its π-orbitals. Thus, the azomethine group with the nitrogen atom has both σ-donor and π-acceptor functions. This provides high stability for metal complexes formed by Schiff bases. Schiff bases can form stable compounds with metal ions if they have a structure that can form a quintet or hexavalent chelate ring [55].

Apart from the fact that the azomethine group is basic, there must be a functional group in the molecule close to the azomethine bond, from which the hydrogen atom can be easily removed, in order to form stable complexes as a ligand. Since Schiff base ligands, which have groups such as OH and SH, in the ortho position adjacent to the azomethine bond, form a six-membered ring with the metal, stable complexes are formed [56].

The properties of complex compounds vary depending on the ligand and metal ion used. The size, charge, and ionization potential of the metal ion used in the formation of the complex affect the stability of the complex. Since the substituents can change the basicity of the Schiff base imine nitrogen, the ligand property also changes depending on the substituents. Therefore, the stability of the metal complexes of Schiff bases is more or less affected by the substituents in their structure.

Three methods are generally used to synthesize metal complexes of Schiff bases. These methods are direct interaction of metal salt with Schiff base [54]; condensation of aldehyde, amine, and metal salt with the effect of template [57]; and condensation of aldehyde complexes with amines [58].

The first studies on Schiff base metal complexes were made by spectrophotometric techniques [59]. Later, potentiometric studies were started by Leussing et al. When these studies were examined, it was seen that Schiff bases formed complexes with metal ions in 1:1 and 1:2 ratios [60].

#### **7.1 Classification of Schiff base complexes**

Classification of Schiff bases metal complexes is done by considering the donor atoms of the compound. Depending on the type and number of donor atoms they contain, some of the metal complexes encountered are those with NdO, OdNdO, OdNdS, NdNdO, OdNdNdO, and NdNdNdN donor atom systems.

### *7.1.1 N*d*O Type Schiff base complexes*

NdO type Schiff base formed by salicylaldehyde and p-N, N<sup>0</sup> -dimethylaniline is bidentate and forms a 1:1 complex with Ag<sup>+</sup> ion (**Figure 14**) [61].

#### *7.1.2 O*d*N*d*O type Schiff base complexes*

[ML]X2 type colored complexes synthesized with (E)–N<sup>0</sup> -((7-hydroxy-4-methyl-2-oxo-2H-chromen-8-yl)methylene)benzohydrazide ligand are examples of OdNdO type Schiff base complexes (**Figure 15**). This ligand forms a complex with metal ions by reacting at a ratio of 2:1. M = Mn(II), Co(II), Ni(II), Cu(II), Sr(II) Cd(II), X = Cl� [62].

#### *7.1.3 O*d*N*d*S type Schiff base complexes*

Schiff base, which consists of 2-hydroxy-1-naphthaldehyde and 2-aminoetantiol, can be given as an example of such complexes. These complexes are tridentate and they have dibasic properties. The structure of the UO2 6+(VI) complex of this Schiff base is shown in **Figure 16** [63].

#### **Figure 14.** *N*d*O type Schiff base metal complex.*

**Figure 15.** *O*d*N*d*O type Schiff base metal complex.*

**Figure 16.** *O*d*N*d*S type Schiff base metal complex.*

*Overview of Schiff Bases DOI: http://dx.doi.org/10.5772/intechopen.108178*

#### *7.1.4 N*d*N*d*O type Schiff base complexes*

Co(II), Ni(II), and Cu(II) complexes of 4-Chloromethyl-2-(2-hydroxynaphthylidenehydrazine) thiazole ligand can be given as examples of such complexes [64] (**Figure 17**).

### *7.1.5 O*d*N*d*N-O type Schiff base complexes*

An example of this group is the oxovanadium(IV) complex synthesized with the N,N0 -bis(salicylidine)ethylenediamine ligand [65]. N,N0 -bis(salicylidine) ethylenediamine Schiff base, which is obtained with the condensation reaction of salicylaldehyde and ethylenediamine, is commonly known as salen and is an ONNO type tetradentate ligand (**Figure 18**) [66].

#### *7.1.6 N*d*N*d*N*d*N type Schiff base complexes*

An example of such complexes is the Fe+2 complex of N,N<sup>0</sup> -bis(2-salicylideniminobenzoyl) ethylenediamine obtained from N,N<sup>0</sup> -bis(2-aminobenzoyl) ethylenediamine and salicylaldehyde (**Figure 19**) [61].
