Question

What is ligand? Explain classification of ligand with illustration.

Answer 1

In coordination chemistry, we come across the term ‘ligand’. These are ions/ molecules with lone pairs of electrons coordinate to the empty d-orbitals of transition metals. They are classified based on the number of binding sites available on ligands.

Complete step-by-step answer:
We must know that a ligand can be as ion or molecule, which has the ability to donate a pair of electrons to the central metal atom or ion of transition metal series to form a coordination complex/coordination entity. ‘Ligand’ is a Latin word, which means “tie or bind”.
Some examples for charged ligands are${{\text{F}}^{\text{-}}}{\text{, C}}{{\text{l}}^{\text{-}}}{\text{, B}}{{\text{r}}^{\text{-}}}{\text{, }}{{\text{I}}^{\text{-}}}{\text{, }}{{\text{S}}_{\text{2}}}^{\text{-}}{\text{, C}}{{\text{N}}^{\text{-}}}{\text{, NC}}{{\text{S}}^{\text{-}}}{\text{, O}}{{\text{H}}^{\text{-}}}{\text{, N}}{{\text{H}}_{\text{2}}}^{\text{-}}{\text{\;}}$ and neutral ligands are${\text{N}}{{\text{H}}_{\text{3}}}{\text{, }}{{\text{H}}_{\text{2}}}{\text{O, NO, CO}}$.
We can classify ligands on the basis of the number of binding sites on the ligand with the central metal atom.
1. Monodentate ligand- It has only one atom that coordinates to the central atom in a complex. For example, ammonia and chloride ions are monodentate ligands.
2. Bidentate ligand- It has two atoms that coordinate directly to the central atom in a complex. An example of bidentate ligand is ethylenediamine.
3. Tridentate ligand- It has three atoms that coordinate directly to the central atom in a complex. An example of tridentate ligand is diethylenetriamine.
4. Tetradentate ligand- It has four atoms that coordinate directly to the central atom in a complex. An example of tetradentate ligand is triethylene tetramine.
5. Hexadentate ligand- It has six atoms that coordinate to the central atom in a complex. An example of hexadentate ligand is ethylene diamine tetra acetate ion [EDTA].

Note:
1. We must be familiar with the term ‘denticity’. It refers to the number of donor groups in a ligand that bind to a central atom in a coordination complex.
2. We must know that the EDTA is the most important chelating agent used in molecular biology. It is extensively used in the analysis of blood. It is an anticoagulant for blood samples, where the EDTA chelates particularly to the calcium present in the blood specimen, arresting the coagulation process and preserving blood cell morphology.