An Overview of Ligands: Understanding Strong and Weak Ligands
Let’s first explore the fundamentals of ligands and their applications before distinguishing between strong ligands and weak ligands.
Definition of Ligand
A ligand is defined as any molecule or atom that binds reversibly to a protein. A single atom or ion can act as a ligand, or it can be a larger, more complex molecule made up of multiple atoms. Ligands can be organic or inorganic molecules found in nature, and they can also be synthesized in a laboratory. This is because a ligand’s chemical structure encompasses all its necessary characteristics. If the lab can replicate this structure, the synthetic ligand will interact in the same way as a natural ligand.
Working of a Ligand
The ligand travels through an organism’s aqueous bodily fluids, such as blood, tissues, or even within the cells themselves. Although the ligand moves randomly, when its concentration is sufficiently high, it will eventually encounter a protein. Proteins that bind to ligands can include receptors, channels, or even the first in a complex series of connected proteins. The binding of the ligand causes the protein to undergo a conformational change. Although no chemical bonds are formed or broken, the physical interaction between the ligand and the protein results in a change in the overall shape of the structure.
What is Strong Ligand and Weak Ligand?
Strong Ligand: A ligand that can induce a greater splitting of the crystal field is referred to as a strong ligand, or a strong field ligand. This means that when a strong field ligand binds, the energy difference between the higher and lower energy orbitals increases. Examples include cyanide ligands (CN⁻), nitro ligands (NO₂⁻), and carbonyl ligands (CO).
Low Spin Complexes
In the formation of complexes with these ligands, the lower energy orbitals (t2g) are completely filled with electrons before any electrons occupy the higher energy orbitals (eg). Complexes formed in this manner are referred to as “low spin complexes.”
Weak Ligand: A weak ligand, also known as a weak field ligand, is one that causes a smaller splitting of the crystal field. This means that the binding of a weak field ligand results in a reduced energy difference between the higher and lower energy orbitals. Examples of weak field ligands include iodide, bromide, and others.
High Spin Complexes
Case, higher energy orbitals can be easily filled with electron rather than lower energy orbitals because there is no repulsion between electron at these energy levels, given the small energy difference between the two orbital level. The complexes formed with these ligands are known as “high spin complexe.”
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 strong ligand or a strong field ligand. | A weak ligand, also referred to as a weak field ligand, is one that induces a smaller 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 | The splitting that occurs after binding with a strong field ligand leads to a larger difference between the higher and lower energy level orbitals. | The splitting of orbitals that occurs when a weak field ligand binds leads to smaller energy differences between the higher and lower energy level orbitals. |
| 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 primary difference between strong and weak ligands is that binding to a strong field ligand causes a larger separation of orbitals, resulting in a greater difference between the higher and lower energy level orbitals, while binding to a weak field ligand leads to a smaller separation and, consequently, a lesser difference.
