Ligands : The Complexity of Coordination Compounds

Ligands compounds

What are Coordination compounds?

Coordination compounds are complex compounds consisting of a central metal atom referred to as Coordination center which is bound to one or more neutral molecules or anions.

What are Ligands?

The molecules or the anions that are bound to a coordination center are called as ligands.

This entity may be an ion as simple as F or a compound such as NH3 or H2O. It can be a large molecule such as NH2CH2CH2NH2 or it can even be a protein (macro-molecule). The properties of coordination compounds are greatly affected by the ions or molecules bonded to it.

Types of Ligands:

Based on the nature o the bond between the ligand and the central atom, they are classified as follows:

  1. Unidentate:

    When the central atom and ligand are bound together through a single donor atom, the ligand is said to be unidentate. Some common unidentates are Cl, H2O etc.

  1. Bidentate:

    If the bonding between the ligand and the coordination centre is through two donor atoms, it is referred as a bidentate.  Oxalate ion (C2O42-) is a bidentate as it can bond through two atoms to the central atom in a coordination compound.

  1. Polydentate:

    Molecules or ions that can bond to a central atom in a coordination compound through two or more donor atoms are known as polydentate ligands. An example of polydentate ligand is Ethylenediaminetetraacetate (EDTA) that can bond to a central atom through six donor atoms.

Chelate Ligand & Denticity

When a polydentate ligand attaches itself to the same central metal atom through two or more (for polydentate) donor atoms, it is known as chelate ligand and the number of atoms that ligate to the metal ion is known as the denticity of such ligands.

Ambidentate Ligand:

The ligands that are capable of bonding to a central atom through two different atoms but only bonds with one of them at a time are called as Ambidentate Ligands. SCN is an example of ambidentate ligand as it can attach itself to a coordination center through nitrogen as well as through sulphur.

The study of central atoms, ligands, and resulting coordination compounds is a wide subject and has proved a milestone in the field of inorganic chemistry.

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Practise This Question

The increasing order of bond dissociation energies of C - C, C - H and H - H bonds is