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Ligands- Definition, Types, Examples, Practice Problems, Frequently Asked Questions

Ligands- Definition, Types, Examples, Practice Problems, Frequently Asked Questions

Feels happy and comfortable amid green plants and trees. You know more than the green colour is of a pigment called chlorophyll and it is vital for the survival of the plants through the photosynthesis process.

How much more do you know about its structure and functions? Chlorophyll has a ”chlorin ring’- a complex cyclic structure with four nitrogen atoms connected differently to Magnesium atoms, as given below. 


What magnesium is doing here? How is it attached to chlorin? What is the mechanism that allows chlorophyll to bind in such a massive structure? You will find answers to all these questions.

Table of Contents

Ligands

Ligands are molecules or ions that use their nonbonding pair electrons to form a coordinate bond with metals. The bonded metal is called the central atom and the whole coordinated species as a coordination entity or molecular/ion.

Simple ions like Cl-, small molecules like H2O or NH3, bigger compounds like H2NCH2CH2NH2 or N(CH2CH2NH2)3, and even macromolecules like proteins are examples of ligands..

Classification of ligands based charge:

Ligands can be classified based on the charge they have- positive ligands, neutral ligands and negative ligands.

Positive Ligands

Neutral Ligands

Negative Ligands

NO+, NO2+ etc

H2O, NH3, CO etc

Cl-, OH-, CN-etc

Denticity

The denticity of a ligand is the number of electron pairs donated by the ligand to form coordinated bonds with the central metal atom.

Classification of ligands based on their denticity:

  • Unidentate Ligands 

Unidentate ligands are ligands that have only one atom that can bind to the coordination centre. Examples of unidentate ligands are ammonia,). Cl-, H2O, OH-, CN-etc.

  • Bidentate Ligands

Bidentate ligands are ligands that can bind to the central atom via two distinct donor atoms present in it.

The ethane-1,2-diamine, oxalate ion and ion dimethyl glyoxime are examples of bidentate ligands because they can attach to the central atom of a coordination complex through two atoms. 


  • Tridentate Ligands

Tridentate ligands are ligands that can bind to the central atom via three distinct donor atoms.

The diethylenetriamine and iminodiacetate are tridentate because they can attach to the central atom of a coordination complex through three atoms.


  • Tetradentate Ligands

Tetradentate ligands are ligands that can bind to the central atom via four distinct donor atoms.

The triethylenetetramine and nitrilotriacetate are tetradentate because they can attach to the central atom of a coordination complex through four atoms.


  • Pentadentate Ligands

Pentadentate ligands are ligands that can bind to the central atom via five distinct donor atoms. 

Ethylenediamine triacetate is pentadentate because it can attach to the central atom of a coordination complex through five atoms. 


  • Hexadentate Ligands

Hexadentate ligands are ligands that can bind to the central atom via six distinct donor atoms.

Ethylenediaminetetraacetate(EDTA) is hexadentate because it can attach to the central atom of a coordination complex through six atoms. 


Flexidentate ligands

Poly-dentate ligands that do not utilize all of their donor atoms to coordinate with the metal ion are known as Flexi-dentate ligands. They use a varied number of donor atoms in various complexes. Simply, a ligand can show flexible denticity.

Example Flexi-dentate ligands include Ethylene diamine tetraacetic acid (EDTA), S2O32-, CH3COO- , SO42- etc.


Ambidentate Ligand

A ligand with two separate donor atoms and either of two donates in the complex is known as Ambidentate Ligand. Example NO, CO, CN-etc.


Chelating Ligand

A ligand that binds to a single metal ion with two or more of its donor atoms which result in a cyclic ring is known as the Chelating ligand.

A chelate complex is the result of this process.

Chelation is the term for the process of chelate production.

For Example DMG, Dimethylglyoxime is forming six-membered rings with Nickel.


Chelating ligands include all polydentate ligands, which can form chelate complexes.

Practice Problems

Q 1. The ligands NH(CH2CH2NH2)2 have denticity 

a. 3
b. 4
c. 5
d. 6

Answer: The diethylenetriamine is tridentate because it can attach to the central atom of a coordination complex through three nitrogen atoms. 

Hence, the denticity is 3. The correct answer is (A).


Q 2. The following ligand is


a. tetradentate
b. Pentadentate
c. Octadentate
d. Hexadentate

Answer: There are three nitrogen and five oxygen atoms in this molecule. As a result, the provided ligand is Octadentate.

As a result, option (C) is the proper response.

Q 3. Why N-(CH2-CH2)3-N can not act as chelating ligands?

Answer: Because N-(CH2-CH2)3-N forms a locked structure, there is ring strain in the structure, and so chelation is not as efficient. Due to the structure's constraints, it's possible that both DABCO (1,4-diazabicyclo[2.2. 2]octane) nitrogens are permanently locked in the structure. As you can see, there is little chance that the metal atom will be off-centre. The metal atom's attempt to fit inside the cage will be sterically hindered. That is why it will not act as chelating ligands.


Q 4. Why H2N-NH2 can not act as chelating ligands?

Answer: However, due to the structure's constraints such as angle strain, the stability of the three-membered ring is very low. So chelation is not as efficient as it is with four-membered (or higher) rings. That is why it will not act as chelating ligands.


Frequently Asked Questions

Q 1. What are the benefits of chelates in the living system?
Answer: Chelates are essential for oxygen transport and photosynthesis. Furthermore, chelates are found in many biological catalysts (enzymes). Chelates are economically important, both as products and as agents in the manufacturing of other compounds, in addition to their importance in living creatures.

Q 2. Why are chelate complexes are more stable than that of monodentate complexes?
Answer: Because the ligand is connected to the metal ion from multiple locations, chelating complexes are more stable than unchelated complexes. As a result, the metal and the ligand are attracted to each other.

Q 3. What makes a ligand particularly effective?
Answer: It depends upon the nature of the donor atom. Weak field ligands bind through relatively electronegative atoms like O and halogens, while strong field ligands bind through C or P. The strength of ligands that bind through N is moderate.

Q 4. How do ligands play an important role in the human body?
Answer: Ligands play an important role in the functioning of the human body. The proteins they regulate might be of many types and functions. Some ligands, like insulin, are employed to signal various things to each cell's metabolism. The brain uses a ligand called acetylcholine to convey nerve impulses between nerves.

Related Topics

Oxidation number of elements in coordination compounds

Crystal field theory

EAN Rule

Bonding in coordination compounds

Organometallic Compounds

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