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Difference Between Strong ligand and Weak Ligand

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Last updated date: 21st Feb 2024
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An Introduction to Ligands: Explaining Strong and Weak Ligand

Lets us understand the basics of ligands and their uses before differentiating strong ligand and weak ligand. 


Definition of Ligand

Any molecule or atom that binds reversibly to a protein is referred to as a ligand. A solitary atom or ion can serve as a ligand. It may also take the form of a bigger, more complex molecule consisting of several atoms. A ligand may be an organic or inorganic molecule found in nature. In a lab, a ligand can also be created synthetically. This is because a ligand's chemical structure contains all of its essential characteristics. The synthetic ligand will be able to interact in the same manner a natural ligand does if that structure can be produced in the lab.


Working of a Ligand

The ligand moves through an organism's aqueous bodily fluids such as blood, tissues, or even within the cell itself. The ligand moves randomly, but when the concentration is high enough, it will ultimately come into contact with a protein. Proteins that take up ligands might be receptors, channels, or even the first in a complicated chain of linked proteins. The protein changes in conformation as a result of the ligand's binding. In spite of the fact that no chemical bonds have been created or broken, the physical action of the ligand interacting with the protein causes the structure's overall shape to change.

What is Strong Ligand and Weak Ligand?

Strong Ligand: A ligand that may cause a greater splitting of the crystal field is said to be strong, also known as a strong field ligand. This indicates that when a strong field ligand binds, the differential between the orbitals with higher and lower energy levels is increased. Cyanide ligands (CN⁻), nitro ligands (NO²⁻), and carbonyl ligands (CO) are a few examples.


Low Spin Complexes

Before any further high energy level orbitals (eg) are filled with electrons during the formation of complexes with these ligands, the lower energy orbitals (t2g) are entirely filled with electrons. "Low spin complexes" are the complexes that are created in this method.


Weak Ligand: A weak ligand, often known as a weak field ligand, is one that can cause less splitting of the crystal field. This indicates that the binding of a weak field ligand results in a smaller difference between the orbitals with higher and lower energies. Iodide, bromide, and other ligands are examples of weak field ligands.


High Spin Complexes

In this instance, higher energy orbitals may be readily filled with electrons compared to lower energy orbitals since there is no repulsion between electrons at those energy levels due to the low difference between the two orbital levels. "High spin complexes" are the complexes created with these ligands.


Difference Between Strong Ligand and Weak Ligand 

Sl.No

Category

Strong Ligand

Weak Ligand

1

Definition

A ligand that can cause a greater crystal field splitting is referred to as a strong ligand or a strong field ligand.

A weak ligand, often known as a weak field ligand, is one that can cause less splitting of the crystal field.

2

Complex involved

"Low spin complexes" are the complexes created with strong field ligands.

"High spin complexes" are the complexes created with weak field ligands.

3

Theory

A greater disparity between the orbitals with higher and lower energy levels results from the splitting after attaching a strong field ligand.

Lower differences between the higher and lower energy level orbitals result from the splitting of orbitals following the binding of a weak field ligand.

4

Magnetism

Most of the complexes that are created are diamagnetic or barely paramagnetic.

Most of the complexes that are created are paramagnetic.


5

Example

Cyanide, Nitro, and Carbonyl Ligands

Iodide, Bromide, and other ligands. 


Summary

The main distinction between strong and weak ligands is that the separation of orbitals following binding to a strong field ligand results in a greater difference between the higher and lower energy level orbitals, whereas the splitting of orbitals following binding to a weak field ligand results in a lesser difference.

FAQs on Difference Between Strong ligand and Weak Ligand

1. What are the characteristics of a strong ligand?

A molecule that possesses a portion of the charge from an atom in its higher-energy configuration is known as a strong field ligand. Compared to this bond's closed form, the partial charges on the strong field ligand's atom create a less favorable environment for interactions that would donate or take away electrons. An interaction with a metal ion in its ground state will be more likely to occur when this ligand is present in the molecule. The ligand in the excited state, will function as an electron donor(Lewis Acid). In both ionic and radical species, Lewis acidity can be produced when the electron-rich electrons from the ligand are supplied to metal ions.

2. What are the characteristics of a weak ligand?

A molecule known as a weak field ligand has partially charged electrons from an atom in a lower energy state, which creates a more conducive environment for electron-donating or withdrawing interactions than the molecule's ground state. These atoms function as uncharged or negatively charged protons in the ground state. Therefore, the area of the molecule without an electron would be more likely to engage a metal ion in its ground state. The ligand will serve as an electron donor when it is stimulated. The ligand's electron-rich electrons can be transferred to metal ions, resulting in Lewis acidity in both ionic and radical species.

3. Explain Strong ligand and Weak Ligand.

The phrase "spectrochemical series" is used to describe how weak field ligands and strong field ligands differ from one another. A ligand with partial charges from an atom is referred to as a weak field ligand. These ligands can function as electron donors in the ground state and nucleophiles in the excited state. The term "strong field ligands" describes the Lewis acid-like ligands that may contribute electron pairs in both their excited and ground states.