Ligand and Its Classification

What is Ligand?

Any neutral or charged species which is capable of donating electrons is known as ligand. It is derived from the Greek word “ligare” which means “to give” i.e. donor. For a species to behave as a ligand, the necessary condition is that it must be nucleophilic in nature. Depending upon the number of donating sites, ligands can be classified as-

(1) Mondodentate Ligands- (Mono = one; dentate from tooth) These are defined as those ligands which can have only one donating site. Some examples are H₃N, H₂O, CO, Cl, Br, I etc. These ligands are also known as the Unidentate ligand.

(2) Bidentate Ligands- (Bi = two; dentate from tooth) These are defined as those ligands which can donate electrons from two positions and hence have two donating sites. Some examples of bidentate ligands are H₂N – CH₂ – CH₂ – NH₂ (Ethylenediamine). It is denoted by (en). Another example of the bidentate ligand is C₂O₄^2- (Oxalate ion).

(3) Polydentate Ligands- These are defined as those ligands which have more than two donating sites. If the number of donating sites is three, four, five, six, then the ligand would be tridentate, tetradentate, pentadentate, hexadentate respectively.

Example of Tridentate Ligand:

Example of Tetradentate Ligand:

Example of Pentadentate Ligand:

Example of Hexadentate Ligand:

Flexidentate Character of Polydentate Ligands:

In certain cases, it is found that all the coordinating sites of a polydentate ligand are not utilized for the formation of a complex. When ligand behaved in such a manner then ligand is showing flexidentate character. Example EDTA is a hexadentate ligand that also acts as tetradentate or pentadentate in certain cases. Therefore ligand (EDTA) in this case show flexidentate character.