Question

How ligands are classified? Explain with suitable examples.

Answer 1

A ligand is an ion or molecule which exists independently of any complex that it might form. In the complex [Co(NH3)6]3+, the ammonia molecules, which can exist outside of the complex, are acting as ligands. The complex is a combination of a Lewis acid (the central metal atom/ion) and some Lewis bases (the ligands).

The main way of classifying ligands is by the number of points at which they are attached to, or bound to, the metal center. This is the denticity. Many ligands are monodentate, but some very important ones are polydentate.

Answer 2

Unidentate ligands: Ligands with only one donor atom, e.g. NH3, Cl-, F- etc.

Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine, C2O42-(oxalate ion) etc. 

Tridentate ligands: Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.

Hexadentate ligands: Ligands which have six donor atoms per ligand, e.g. EDTA. 

Chelating Ligands: Multidentate ligand simultaneously co-ordinating to a metal ion through more than one site is called chelating ligand. These ligands produce a ring like structure called chelate. Chelation increases the stability of complex. This effect is called chelation effect.

Example:

                         ••                            _
          H2C — NH2           O = C — O :
            |       ••                   |       _
          H2C — NH2           O = C — O :