Ligands – Definition, Types, Function and Examples

Ligands: Ligands are those molecules or ions in a coordination compound that donate a pair of electrons to the central atom of the compound. Ligands can be positively charged, negatively charged or neutral metal atoms and help in binding the molecules together. The term ‘Ligand’ is an English word derived from a Latin word Ligandus which means ‘to bind’. In most cases the ligands are found to form a covalent bond with the central atom.

Some examples of ligands are NO⁺, N₂H5⁺ (positively charged), Br^–, I^–, S₂^– (negatively charged) or H₂O, NO, CO (neutral atoms). The ligands are classified on the basis of number of binding sites, size of the binding atom and the charge they carry. the concept of ligands is very crucial to understand the important chemical reactions studied in Inorganic and Organic chemistry. The students can find the complete details of the same in this article. Continue reading.

What is Ligand?

Ligands may be defined as molecules or ions with lone pair of electrons that attach themselves with the central metal atom or ions with high positive charge density. It can be inferred that ligands are Lewis base as they donate electrons, and the central metal atom or ion is Lewis acid as they accept electrons. Based on the number of binding sites with the central metal atom or ion, charge and size ligands are classified into different types.

Examples of some common neutral ligands are water \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right),\) ammonia \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right){\rm{.}}\) Cyanide (\(\left( {{\rm{C}}{{\rm{N}}^{\rm{ – }}}} \right){\rm{,}}\) chloride \(\left( {{\rm{C}}{{\rm{l}}^{\rm{ – }}}} \right)\) are some anionic ligands and some ligands are cationic like \(\left( {{\rm{N}}{{\rm{O}}^{\rm{ + }}}} \right)\) and are electron-pair acceptors. Based on the attachment of the ligand to the center, they are classified as monodentate ligand (metal may be attached through a single atom), bidentate or polydentate ligand (metal may be attached through two or more atoms).

Ligands Examples

Ligands are neutral, anionic, or cationic. Examples of some common neutral ligands are water \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right),\) ammonia \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right){\rm{.}}\) Cyanide \(\left( {{\rm{C}}{{\rm{N}}^{\rm{ – }}}} \right){\rm{,}}\) Chloride \(\left( {{\rm{C}}{{\rm{l}}^{\rm{ – }}}} \right),\left( {{\rm{B}}{{\rm{r}}^{\rm{ – }}}} \right){\rm{,}}\) and \(\left( {{\rm{O}}{{\rm{H}}^{\rm{ – }}}} \right)\) are some of the anionic ligands and some ligands are cationic like \(\left( {{\rm{N}}{{\rm{O}}^{\rm{ + }}}} \right){\rm{,}}\left( {{{\rm{N}}_{\rm{2}}}{\rm{H}}_{\rm{5}}^{\rm{ + }}} \right)\) and they are electron-pair acceptors.

Denticity of Ligands

The word denticity is derived from the Latin word dentist which means tooth. We can imagine that a ligand bites the central metal at one or more linkage points. Denticity of Ligands is the number of coordinating or donor groups present in a single ligand that facilitates them to bind with the central metal atom or ion. Ligands are classified into different types based on denticity.

Classification of Ligands

Based on the denticity or number of sites, ligands are classified as monodentate, bidentate or polydentate, ambidentate, bridging ligands. Let’s discuss them in detail to understand ligands types.

1. Monodentate Ligands

As the name signifies, ‘mono’ means ‘one’ and ‘dentate’ means ‘toothed’; monodentate ligands are ligands that bind to the central atom through one atom only.

2. Didentate or Bidentate Ligands

Didentate or Bidentate ligands are molecules or ions that bind to an atom via two coordinate covalent bonds. In short, ligands having two donor sites are known as Didentate or Bidentate Ligands. Examples of some bidentate ligands are given below:

3. Tridentate and Polydentate Ligands

Ligands with three lone pairs of electrons to donate to the central metal atom or ion are called tridentate ligands, ligands with four donor atoms are called tetradentate, ligands with five donor atoms are called pentadentate, ligands with six donor atoms are called hexadentate and so on. Hence, collectively such ligands with more than two donor sites are termed polydentate ligands.

4. Ambidentate Ligands

As the name signifies, any word having the prefix-‘Ambi’ is derived from a Latin word ambiguous meaning ‘both’. The ligands that can bind with the central atom or ion via two different elements in two separate ways are known as ambidentate ligands. For example, when nitrite ion binds with a central metal atom or ion via \({\rm{N – }}\)atom, it is known as ‘nitro’ ligand, and when it binds with \({\rm{O – }}\)atom, it is known as ‘nitrite ligand.

5. Bridging Ligand

Ligands that connect two or more central atoms or ions are known as bridging ligands. They are differentiated in nomenclature by the prefix \({\rm{‘\mu ‘}}{\rm{.}}\) Bridging ligands are primarily used in coordination polymers. For example, Carbonyl \({\rm{(CO)}}\) forms \({\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{(CO)}}_{\rm{9}}},\) Hydride \(\left( {{{\rm{H}}^{\rm{ – }}}} \right)\) form \({{\rm{B}}_{\rm{2}}}{{\rm{H}}_{\rm{6}}}{\rm{,}}\) etc.

Chelate Effect

The term ‘chelate’ is derived from the Greek word ‘close’ that means ‘crab’. The ligand, which is attached to a central metal atom or ion by bonds from two or more donor atoms, is a chelating ligand. The primary difference between a chelating ligand and a polydentate ligand is that a chelating ligand bind more than once to the same central metal atom, but a normal polydentate ligand does not necessarily ligate to the same metal.

Therefore, a polydentate ligand doesn’t need to be a chelating ligand. The chelate effect is defined as the amplified affinity of a chelating ligand for a central metal ion compared to its monodentate non-chelating ligands.

For example, a complex containing chelate rings such as \({\left[ {{\rm{Co}}{{({\rm{en}})}_3}} \right]^{3 + }}\) is more stable than \({\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]^{3 + }}\) where ethylenediamine \(\left( {{\rm{en}}} \right)\) is an example of a chelating ligand.

Spectrochemical Series of Ligands

Crystal field splitting occurs due to the field produced by ligands on the central metal atom. Thus a series of absorption spectra caused due to absorption of light as a result of this splitting of energy levels is known as spectrochemical series.

Based on the result of absorption spectra of cobalt complexes, a spectrochemical series was found in \(1938.\) It is a list of ligands arranged in a series according to their field strength, i.e. based on the splitting of energy levels. Based on the splitting magnitude, ligands are classified into two types:

1. Strong Field Ligands – Ligands that cause large splitting of energy levels. They form low spin complexes. For example \({\rm{CO,C}}{{\rm{N}}^{\rm{ – }}}\) etc.

2. Weak Field Ligands – Ligands that cause less or small splitting of energy levels. They form low spin complexes. For example \(\left( {{\rm{C}}{{\rm{l}}^{\rm{ – }}}} \right){\rm{,}}\left( {{\rm{B}}{{\rm{r}}^{\rm{ – }}}} \right),\) etc.

Given below the spectrochemical series of ligands:

Summary

It can be concluded that ligands are the building blocks of coordination compounds. In coordination, a chemistry ligand is a molecule or ions that bind with the central metal atom and, as a result, form a coordination complex. It acts as an electron-pair donor and the central atom act as an electron pair acceptor. At least one donor atom with an electron pair should be present in a ligand that is used to form a covalent bond with the central atom. Coordination compounds based on the presence of transition metal and type of ligands are used in the dye and pigment industry.

Haemoglobin is a famous example of a coordination complex. Students can refer to the Ligands table to have a quick review of the concept. Thus we have come to know about ligands, their types based on denticity, spectrochemical series, and many more.

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