Ligands

Ligands are ions or molecules that form a bond with the central atom of a coordination compound by donating its pair of electrons to it. A ligand may be positively charged, negatively charged, or neutral. Ligands act as electron-donating species and can be called Lewis bases whereas the central atom acts as an electron pair acceptor and hence be called a Lewis Acid.

In this article, we will discuss the definition of a ligand, various types of ligands, and their examples.

What is a Ligand?

Ligands are chemical species that come under a branch of chemistry called coordination chemistry. The word ligand comes from Latin, which means “tie or bind”. In a coordinate bond, the ligand donates its pair of electrons to the central atom or ion to form a coordination compound. Thus, ligands act as electron-donating species and can be called Lewis bases. Ligands have at least one donor atom with an electron pair used to form covalent bonds with the central atom.

Ligand Definition

Ligand can be defined as an ion or molecule which donates its electron pair to another ion or atom (called as the central atom) to form a coordinate bond in a coordination compound.

Owing to the electron-donating nature of a ligand, it can also be classified as a Lewis base. A ligand can be an anion, a cation, or a neutral chemical species.

Ligand Examples

Ligand depends on the number of lone pair that binds the central atom. Some examples of ligands include:

• Fluoride ions
• Chloride ions
• Bromide ions
• Iodide ions
• Oxalate ions
• Diethylenetriamine
• Acetylacetonate ions
• EDTA, etc

Strong and Weak Ligands

Strong Ligands also known as Strong Field Ligands are the ligands that can cause a greater splitting in the crystal field and have a low spin value, on the other hand, weak field ligands or weak ligands cause a lower splitting of the crystal field and have a high spin value.

Examples of Strong and Weak Ligands

OH^–, CN^–, and NH₃ are a few examples of strong ligands, and H₂O, F^–, and Cl^– are examples of weak ligands.

Classification of Ligands

Ligands are classified on the basis of the number of donor groups in them that bind to the central metal atom in forming a coordination compound. This property of ligands is called the denticity of a ligand. Thus, the Denticity of a Ligand can be defined as the number of donor sites in a ligand that get connected to a central atom in the coordination compound. Types of Ligands are as follows:

• Monodentate Ligands
• Bidentate Ligands
• Tridentate Ligands
• Polydentate Ligands

Monodentate Ligands

The ligands having only one donor site are classified as monodentate ligands. These are also called “one-toothed” ligands as they bind the central atom at only one place.

Below are some examples of monodentate ligands:

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

Bidentate Ligands

The ligands which donate two lone pairs of electrons to the central atom in forming a coordination compound are known as bidentate ligands. These ligands have more affinity towards the central atom as they have more than one donating site.

Examples of Bidentate Ligands

One of the main examples of bidentate ligands is ethylenediamine in which both nitrogen atoms get involved in bond formation simultaneously. Other examples include oxalate ion, acetylacetonate ion, phenanthroline, etc.

Bidentate ligands are also called “chelating ligands” and the coordination compounds formed by them are called “chelates“. Also, there is something known as a chelate effect which talks about the improved affinity of bidentate ligands for the central atom in comparison to that of monodentate ligands for the same central atom.

Tridentate Ligands

The ligands having three lone pairs of electrons to donate to the central atom in the formation of a coordination compound are called tridentate ligands.

Example of Tridentate Ligands

Diethylenetriamine is an example of a tridentate ligand in which three nitrogen can donate electrons to the central atom forming a coordination compound.

Polydentate Ligands

The ligands having more than one donating site or a number of lone pairs of electrons are collectively called polydentate Ligands. Polydentate ligands are classified as below:

Tetradentate Ligands: The ligands having four lone pairs to donate are called tetradentate ligands.

Pentadentate Ligands: The ligands having five lone pairs are called pentadentate ligands.

Hexadentate Ligands: The ligands with six lone pairs are termed hexadentate ligands, and so on.

Below are some examples of polydentate ligands along with their denticity:

Polydentate Ligand	Name of the Ligand	Abbreviation	Denticity
Tridentate Ligand	Diethylenetriamine	DETA	3
Tetradentate Ligand	Triethylenetetramine	trien	4
Hexadentate Ligand	Ethylenediaminetetraacetate	EDTA	6

Chelating Ligands

Chelating Ligands have two or more donor sites which form a coordination bond with the central atom. Bidentate or polydentate ligands are termed under chelating ligands, therefore, chelating ligands are also known as multidentate ligands.

Examples of Chelating Ligands

Here are a few examples of Chelating Ligands:

• Ethylenediamine (en)
• Acetylacetone (acac)
• Ethylenediaminetetraacetic acid (EDTA)
• Oxalate, etc.

pi-acceptor Ligand

pi-acceptor Ligands can accept electrons from a metal into the empty pi-orbital. These ligands can also donate electrons from a σ orbital. Examples of pi-acceptor ligands are CO, NO⁺, NH_3,polydentate ligands etc.

Bipyridine Ligand

Bipyridine or 2, 2′-bipyridine is an organic compound whose chemical formula is C₁₀H₈N₂. It is a chelating ligand that can form complexes with many transition metals. It act as polydentate ligands and is also termed as neutral ligands.

Ligand Important Terms

Some of the important terms related to ligands are discussed below:

• Chelate Effect
• Ambidentate Ligands
• Bridging Ligands
• Coordinate Compounds
• Coordination Number
• Denticity

Chelate Effect

Chelate effect attributes to the property of polydentate ligands which allows them more efficient bonding with the central atom as there is more than one donor site in these ligands as compared to monodentate ligands which have only one donor site for bond formation. Thus, these ligands are also known as chelating ligands and the coordinate compound formed is known as the chelate.

In other words, it can be said that the effect which explains the improved affinity of chelating ligands towards the central atom than monodentate ligands is known as the chelate effect.

Ambidentate Ligands

Ambidentate ligands are those ligands which have more than one potential donor site for bond formation but at a particular time, only one of them takes part in bond formation. The choice of the donor site depends on some certain external factors.

For example, SCN^– is an ambidentate ligand which has two donor sites a Nitrogen atom and a Sulphur atom, but only one of them takes part in bond formation at a particular time.

Bridging Ligands

Bridging Ligands are the ligands in which the donor atom makes coordinate bond with more than two other atoms. These are called so as structure of the coordination compound formed by them is similar to a bridge with the metal atoms at end and the ligand in between them. The donor atom in bridging ligands has at least two lone pairs. Some examples of bridging ligands are OH⁻, S₂⁻, NH₂⁻, CO, etc.

Coordinate Compounds

Coordinate compounds are a class of chemical compounds in which a central metal atom or ion is linked to surrounding molecules through a coordinate bond. A coordinate bond is formed by the complete transfer of electrons from the donor atom to the central atom. Central metal atom is also called as Lewis acid and the donor element is called as Lewis base. The donor element is termed as the ‘Ligand’.

An example of coordinate compounds is [Fe(CN)₆]^4-. Here, Fe is the central metal atom and CN^– is the ligand.

Denticity

Denticity is the number of donor groups in a ligand that binds to a central metal atom in a coordination complex. It determines the bonding of a Ligand. Denticity of a ligand can be determined by its coordination number.

Coordination Number

The total number of donor atoms (Ligands) attached to the central metal atom is known as its coordination number. It is also known as ligancy.

Ligand List

Let us take a look at some of the common ligands, their symbols, and an example of coordination compound formed by them in the form of the following table:

Ligand Name	Symbol/Formula	Example
Bromide ion	Br–	[Cu(Br)4]2-
Ammonia	NH3	[Pt(NH3)6]4+
Cyanide ion	CN–	[Fe(CN)6]4-
Carbon monoxide	CO	Ni(CO)4
Chloride ion	Cl–	[PtCl4]2–
Hydroxide ion	OH–	[Zn(OH)4]2-
Oxalate ion	(C2O4)–	[Fe(C2O4)3]3-
Ethane-1,2-diamine	en	[Co(en)3]3+

Ligand Charge Table

Below is a table discussing some ionic ligands along with the charge on them.

Ligand	Symbol	Charge
Chloride ion	Cl–	-1
Cyanide ion	CN–	-1
Hydroxide ion	OH–	-1
Thiocyanate ion	SCN–	-1
Sulfate ion	SO42-	-2
Carbonate ion	CO32-	-2
Oxide	O2-	-2
Amide ion	NH2+	+1
Nitronium ion	NO2+	+1

Read More,

• Coordinate Compounds
• Crystal Field Theory
• Werners Theory

Ligands Frequently Asked Questions

What are Chelating Ligands?

Chleating Ligands have two or more donar sites which form a coordination bond with the central atom.

What is Denticity of Ligand?

Denticity of a Ligand can be defined as the number of donar sites in a ligand that get connected to a central atom in the coordination compound.

What are Ambidentate Ligands?

The ligands which have more than one potential donor site for bond formation with central atom but at a certain time only one of them is involved in bond formation are known as ambidentate ligands.

What are Bridging Ligands?

The ligands which can bind to more than one metal atom or ion are called bridging ligands. Chloride ion, Hydroxide ion, Nitride ion, etc. are some examples of bridging ligands.

What is Chelate Effect?

‘Chelate effect’ is a term used for chelating ligands as they have increased affinity for the central metal atom or ion in comparsion to monodentate ligands as they have two donor sites instead of one.

What is Coordination Number in Ligand?

The total number of donor atoms (Ligands) attached to the central metal atom is known as the coordination number.

What are Three Types of Ligands?

Ligands are classified into three types on basis of charge on them. These are Anionic Ligands, Cationic Ligands and Neutral Ligands. For example, F^–, Cl^–, Br^–, OH^–, etc. are anionic ligands, NH₂⁺, NO₂⁺ etc. are cationic ligands, and H₂O, NH₃, etc. are neutral ligands.

Is Ligand a metal?

No, Ligands are ions or neutral molecules which bind to a central metal atom to form a coordination compound.

What is Charge of Ligand?

A Ligand can be positively charged, negatively charged or neutral molecule. The charge on the molecule is called the charge of a ligand. For example, charge of the ligand OH^– is -1.

What is Ligand in Chemistry and what is its Purpose?

In chemistry, a ligand is an ion or a neutral molecule having lone pair of electrons which it can donate to a central metal atom to form a coordinate bond and thus forming a coordinate compound.

What is an Example of Ligand?

Ethylenediamine (H₂NCH₂CH₂NH₂) is an example of a ligand. It is a neutral ligand which means that the charge on it is zero.

Is Oxygen a Ligand?

Yes, oxygen is a ligand. The coordinate compounds formed by oxygen as a ligand are known as dioxygen complexes. Myoglobin and Haemoglobin are examples of coordinate compounds containing oxygen as a ligand.