Ligands: Ambidentate Ligand, Chelate Effect, Types of Ligands

Ligands are ions or molecules capable of donating a pair of electrons to the central metal atom and in doing so form a coordination compound. Ligands play a very important role in Coordination Chemistry. Ligands usually act as electron-pair donors (Lewis Bases) and the central atom acts as electron-pair acceptors (Lewis acids). A coordination compound consists of a central atom or ion, which is usually a transition metal surrounded by a group of similar or different ions or neutral molecules called ligands. 

Key Terms: Ligander, ambidentate ligand, chelating ligand, bidentate ligand, chelate effect, monodentate ligand

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Coordination Compounds and Ligands 

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These are platinum, cobalt, and other transition metal compounds that are made up of two parts: a core atom and ligands. Co-ordination bonds are used to connect the ligands to the central atom. An atom or a group of atoms (called ligands) is/are bonded to the central atom in these compounds by using a shared pair of electrons supplied by the coordinated group rather than the central atom. The empirical formulae and characteristics of these substances are unusual. 

It's not unusual for them to be brightly coloured compounds. Their most distinguishing feature is the presence of two, four, six, or even more chemical groups arranged geometrically around the metal ion (also known as the central atom). These molecules (also known as ligands) can be either neutral, cations, or anions. Each co-coordinating group can exist on its own or be connected together in a single long, flexible molecule that wraps around the metal. Co-ordinating groups have a significant impact on a metal's chemical behaviour.

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Features of Ligands

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There are a few conditions that must be met for an atom, a group of atoms, or an ion to act as a ligand. These are the following:

• At least one lone pair of electrons should be present in ligands.
• Ligands should be able to transfer their lone pair of electrons to the central metal atom or ion in order to form covalent bonds with it.
• The Ligand acts as a Lewis base, while the metal atom or ion acts as a Lewis acid, and the two react to generate a co-ordination complex by a Lewis acid-base reaction.

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Types of Ligands

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Coordination Compounds have two parts, the central metal atom or ion and the ligand ions or molecule surrounding it. Ligands can be classified based on several properties. Some of which are discussed below:

Based on Charge

On the basis of charge carried by the ligand, it is classified into 3 different types.

• Neutral Ligands: Ligands that do not have any charge on them are known as neutral ligands. 

Example: H₂O, NO, CO, C₆H₆, etc.

• Positive Ligands: Ligands that carry at least one positive charge are known as positive ligands.

Example: NO⁺, NH^2-NH³⁺, etc.

• Negative Ligands: Ligands that carry at least one negative charge are known as negative ligands.

Example: Cl^-, NO^2-, CN^-, OH^-, etc.

Based on Denticity

The number of donations accepted by a central metal atom from a particular ligand is known as the denticity of the ligand. Ligands may be classified as follows based on their denticity:

1. Monodentate Ligands: Ligands from which only one donation is accepted by the central atom are called monodentate ligands.

Examples: 

2. Bidentate Ligands: Ligands from which two donations are accepted by the central atom are called bidentate ligands.

Example: 

3. Tridentate Ligands: Ligands from which three donations are accepted by the central atom are called tridentate ligands.

Examples: 

4. Tetradentate Ligands: Ligands from which four donations are accepted by the central atom are called tetradentate ligands.

Examples:

5. Pentadentate Ligands: Ligands from which five donations are accepted by the central atom are called pentadentate ligands.

Examples: EDTA^3- - ethylenediamine triacetate

6. Hexadentate Ligand: Ligands from which six donations are accepted by the central atom are called hexadentate ligands.

Examples: 

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Based on Bonding Interaction

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On the basis of bonding interaction between the ligand and the central atom, ligands can be classified into two.

1. Classical or Simple Donor Ligands: These ligands only donate the lone pair of electrons to the central atom.

Example: O^2-, OH^-, F^-, NH^2-, NH₃, N^3-, etc.

2. Non-Classical Ligands: These ligands not only donate the lone pair of electrons to the central atom but also accept the electron cloud from the central atom in their low-lying vacant orbitals. This back donation of electrons is called synergic bonding.

Example:  

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Chelation Effect

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Chelation is a type of bonding existing between ions or other species and the central atom. It involves the formation of one or more coordinate bonds between the ligands and the single central metal atom. The resulting complex compound has a ring-like structure.

• Ligands that can form a ring-like structure with the central atom is called a chelating ligand. 
• All polydentate ligands are examples of chelating ligands.
• Chelated complexes are more stable than complexes having monodentate ligands as dissociation of chelated complexes involves the breaking of two bonds instead of one.
• NH₂NH₂  cannot act as a chelating ligand because it forms a three-membered ring.

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Ambidentate Ligand

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A ligand that may have more than one kind of donor site but at a time only one kind of donor site is utilized for donation is called ambidentate ligand. Ambidetate ligands are further classified into two categories:

1. Monodentate and Ambidentate Ligands
2. Bidentate and Ambidentate Ligands

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Flexidentate Ligand

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A ligand that shows variable denticity is known as the flexidentate ligand. Usually, sulphates and carbonates act as flexidentate ligands.

Example:

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Bridging Ligands

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Bridging ligands are those that are bound to more than one metal atom.

Bridging ligand is represented by “mu, \eta” which relates to hapticity. Below is the list of bridging ligands:

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Things To Remember

• Coordination compounds have two parts: the central atom and surrounding ligands.
• Ligands act as lewis base.
• The central atom acts as lewis acid.
• Ligands are classified on the basis of 3 parameters: Charge, Denticity, Bonding Interaction.
• On the basis of charge, Ligands are of 3 types: Neutral, Positive, and negative.
• On the basis of Denticity, Ligands are of 6 types: Monodentate, Bidentate, Tridentate, Tetradentate, Pentadentate, Hexadentate.
• On the basis of bonding interaction between ligand and central atom, Ligands are of two types: Classical and Non-classical.

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