Ligands

What Are Different Types of Ligands?

A ligand is an ion or molecule which donates a pair of electrons to the central metal atom or ion to form a coordination complex. The word ligand is from Latin, which means “tie or bind”. Ligands can be anions, cations, and neutral molecules. Ligands act as Lewis bases (donate electron pairs), and central metal atoms are viewed as Lewis acids (electron pair acceptors). The nature of bonding from metal to ligand varies from covalent bond to ionic bond.

Occasionally ligands can be cations (NO⁺, N₂H5⁺) and electron-pair acceptors. Examples for anionic ligands are F^–, Cl^–, Br^–, I^–, S₂^–, CN^–, NCS^–, OH^–, NH₂^– and neutral ligands are NH₃, H₂O, NO, CO.

A ligand is an ion or molecule which binds to the central metal atom to form a coordination entity or complex compounds. Classification of ligands is on the basis of the number of binding sites with the central metal atom, charge and size.

Also Read: Coordination compound

Monodentate Ligands

Monodentate ligands are also called “one-toothed“ because they bite the metal atom only in one place.

Ligand	Lewis Structure	Name
Br–		Bromide ion
F–		Fluoride ion
Cl–		Chloride ion
I–		Iodide ion
H2O		Water
NH3		Ammonia
OH–		Hydroxide
CO		Carbon monoxide
CN–		Cyanide ion
SCN–		Thiocyanate ion

Bidentate Ligands

Lewis bases, which donate two lone pairs of electrons to the central metal atom, are known as bidentate ligands. They are often referred to as “chelating ligands”. The complex which contains chelating ligands is called “Chelates”.

	Ethylenediamine (en)
	Acetylacetonate ion (acac)
	Phenanthroline (phen)
	Oxalate ion (ox)

Tridentate Ligands and Polydentate Ligands

Tridentate ligands have three lone pairs of electrons to the central metal atom or ion. Molecules with four donor atoms are called tetradentate, five donor atoms are called pentadentate, and six donor atoms are called hexadentate. They are generally mentioned as polydentate ligands.

Chelate Effect

The chelate effect explains the enhanced affinity of chelating ligands for central metal ions or atoms compared to the affinity of nonchelating monodentate ligands for the same metal.

Ambidentate Ligands

Ligands with more than one potential donor atom are known as ambidentate ligands. For example, thiocyanate ion(NCS^–) which can bind to the central metal atom or ion with either nitrogen or sulfur atoms.

Bridging Ligands

A bridging ligand is one which is bound to more than one metal atom.

The bridge ligand is preceded by “mu,\eta”, which relates to hapticity. The list of bridging ligands is as follows:

Organic ligands – Organic derivatives of above-mentioned inorganic ligands, and they form a strong bridge between metal centres.

Note: Hapticity is the number of atoms of the ligand which are bound to the central metal atom.

Summary

• Ion or molecule capable of donating a pair of electrons to the central metal atom via donor atom.
• Ligands are classified based on the number of lone pair electrons available for the central metal atom, size and charge like anionic, cationic, neutral, monodentate, bidentate, and polydentate ligands.

Frequently Asked Questions

1. What type of ligand is EDTA?

Answer:

EDTA is an example of a hexadentate ligand. It binds four times at the oxygen atom and twice at nitrogens. EDTA is usually used in the form of salt or dry form. EDTA is a great chelating agent which forms multiple bonds in coordination compounds.

2. Is NH₃ an example of a monodentate ligand?

Answer:

Ammonia has lone pair of electrons that is shared with a central metal atom or ions. NH₃ is capable of sharing only one pair of electrons, so it is an example of a monodentate ligand.

3. Is oxygen a ligand?

Answer:

Dioxygen complexes are an example of coordination compounds which contain O₂ as ligands. Many transition metal form complexes with O_2.