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 viewed as Lewis acid (electron pair acceptor). The nature of bonding between 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

Mono dentate 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 Bromide ion
F Fluoride ion Fluoride ion
Cl Chloride ion Chloride ion
I Iodide ion Iodide ion
H2O Water Water
NH3 Ammonia Ammonia
OH Hydroxide ion Hydroxide
CO Carbon Monoxide Carbon monoxide
CN Cyanide ion Cyanide ion
SCN Thiocyanate ion Thiocyanate ion

Bidentate Ligands

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

Ethylenediammine Ethylenediammine (en)
Acetylacetonate ion Acetylacetonate ion (acac)
Phenanthroline Phenanthroline (phen)
Oxalate ion (ox) 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.

Polydentate Ligands

Chelate Effect

Chelate effect explains the enhanced affinity of chelating ligands for central metal ion or atom compared to the affinity of nonchelating monodentate ligands for the same metal.

Ambidentate ligands

Ligands with more than one potential donor atoms 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

Bridging ligands is the one which is bound to more than one metal atom.

Bridging ligands

Bridge ligand is preceded by “mu, \eta “ which relates to hapticity. List of bridging ligands are as follows:

Inorganic bridging ligand Name Example
OH Hydroxide [Fe2(OH)2(H2O)8]4+
N3- Nitride [Ir3N(SO4)6(H2O)3]4-
CO carbonyl Fe2(CO)9
Cl Chloride Nb2Cl10
NH2- Amido HgNH2Cl
H- Hydride B2H6
O2 Oxide [Cr2O7]2-

Organic ligands – A organic derivatives of above mentioned inorganic ligands and they forms a strong bridge between metal centers.

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, polydentate ligands.

Frequently Asked Questions

1. EDTA is what type of ligand?

Answer:

EDTA is an example of hexadentate ligands. It bindes four times at 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 is an example of monodentate ligand?

Answer:

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

3. Is oxygen is a ligand?

Answer:

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