Introduction

A complex compound is one in which two or more anions or neutral molecules are coordinated by a metal atom or ion. A coordination complex contains one or more complicated ions in the majority of cases. K₄[Fe(CN)₆] , for example, is composed of the complex ion  [Fe(CN)₆]⁴⁺. During the creation of a complex, a ligand is a chemical or ionic species that forms a direct bond with the central metal atom or ion. The ligands and the central atom or ion are linked via coordinate bonds. As a result, the coordinating group, molecule, or ion in a complex ion is frequently referred to as the ligand.

Polydentate ligands are those that coordinate with the central ion via more than two donor atoms in the molecule. They can be tridentate, pentadentate, or hexadentate, with three, five, or six donor sites respectively.

Chelating agents are found in all polydentate ligands. Chelates are more stable than comparable non chelated compounds, and the more extensive the chelation—that is, the greater the number of ring closures to a metal atom—the more stable the complex.

What is Ligand?

1. A ligand is a chemical or ionic species that becomes directly linked to the central metal atom or ion during complex formation.
2. Coordinate bonds connect the ligands to the central atom or ion. As a result, ligands are sometimes referred to as the coordinating group, molecule, or ion in a complex ion.
3. The atom in the ligand that can give the electron pair is referred to as the donor atom or the coordinating atom. In ammonia, for example, nitrogen is the donor atom, whereas in water, oxygen is the donor atom.
4. To form a complex, electron pairs given by ligands are accommodated in empty orbitals of the central metal atom.

Types of Ligands

One or more donor sites can be detected in a ligand. Ligands can be one of the following categories, depending on the number of donor sites:

1. Monodentate or unidentate ligands
2. Bidentate ligands
3. Polydentate ligands 

A ligand is said to be monodentate if it can only establish one coordinate covalent bond per molecule. If there are two donor sites, the ligand is a bidentate ligand. This classification is described by the denticity of ligands. Because the ligand in a chelate is linked to the central metal atom via two or more donor sites, the ligand is either bidentate or polydentate. The ligand of a chelate is usually cyclic or ring in structure. Chelating agents are another name for these ligands.

Polydentate ligands

1. Polydentate ligands are those that coordinate with the central ion via more than two donor atoms in the molecule.
2. They can be tridentate, pentadentate, or hexadentate, with three, five, or six donor sites.
3. Ethylenediamine tetraacetic acid (EDTA), for example, is a significant hexadentate ligand. The EDTA ion bonds to the central metal ion via two nitrogen and four oxygen atoms of the four COOH groups.
4. Other significant polydentate ligands include: Terpyridine, Diethylenetriamine

Is it true that all polydentate ligands chelate?

Chelating agents are found in all polydentate ligands. Chelates are more stable than comparable non chelated compounds, and the more extensive the chelation—that is, the greater the number of ring closures to a metal atom—the more stable the complex. 

 Conclusion

We can conclude that ligands are the basic building blocks of coordination complexes. A chemical ligand in coordination is a molecule or ion that forms a coordination compound with the central metal atom. It serves as an electron pair donor, whereas the central atom serves as an electron pair acceptor. Polydentate ligands are those that coordinate with the central ion via more than two donor atoms in the molecule. They can be tridentate, pentadentate, or hexadentate, with three, five, or six donor sites. Chelating agents are found in all polydentate ligands. Chelates are more stable than comparable non chelated compounds, and the more extensive the chelation—that is, the greater the number of ring closures to a metal atom—the more stable the complex.