The key difference between ligand and chelate is that ligands are the chemical species that donate or share their electrons with a central atom through coordination bonds, whereas chelates are compounds containing a central atom bonded with surrounding ligands.
We can define a ligand as an atom, ion, or molecule that can donate or share two of its electrons through a coordination covalent bond with a central atom or ion. Similarly, we can define a chelate as a compound consisting of a central metal atom bonded to a ligand having at least two or more donor sites.
CONTENTS
1. Overview and Key Difference
2. What is a Ligand
3. What is a Chelate
4. Side by Side Comparison – Ligand vs Chelate in Tabular Form
5. Summary
What is a Ligand?
A ligand is an atom, ion, or molecule that can donate or share two of its electrons through a coordination covalent bond with a central atom or ion. Usually, we talk about ligands under the field of coordination chemistry.
According to the crystal field theory, there are two types of ligands as strong ligands and weak ligands. A strong ligand or a strong field ligand is a ligand that can result in a higher crystal field splitting. This means the binding of a strong field ligand causes a higher difference between the higher and lower energy level orbitals. Examples include CN– (cyanide ligands), NO2– (nitro ligand) and CO (carbonyl ligands). A weak ligand or a weak field ligand is a ligand that can result in a lower crystal field splitting. This means the binding of a weak field ligand causes a lower difference between the higher and lower energy level orbitals.
Moreover, we can divide ligands into groups based on the chemical structure, such as macrocyclic ligands. A macrocyclic ligand is a large cyclic structure having three or more donor sites. A macrocyclic ligand is essentially a large cyclic structure. There are at least three or more donor sites in a macrocyclic ligand. These ligands show a very high affinity for metal ions.
What is Chelate?
A chelate is a compound consisting of a central metal atom bonded to a ligand having at least two or more donor sites. Therefore, the chelate is the whole complex that contains the central metal atom and the ligand. We can name this complex also as a coordination complex or coordination compound. Some coordination compounds have two or more ligands bonded to the central metal atom, but a chelate has only one ligand.
There are several categories of ligands. Their name indicates how many coordinate covalent bonds they can form. For example, if a ligand can form only one coordinate covalent bond per molecule, it is known as a monodentate ligand. Likewise, if there are two donor sites, then it is a bidentate ligand. The denticity of ligands describes this categorization. Since the ligand is attached to the central metal atom via two or more donor sites in a chelate, the ligand is either a bidentate or a polydentate ligand. Most of the times, the ligand of a chelate is a cyclic or a ring structure. These ligands are also known as chelating agents.
What is the Difference Between Ligand and Chelate?
The terms ligand and chelate are highly related to each other. The key difference between ligand and chelate is that ligands are the chemical species that are donating or sharing their electrons with a central atom through coordination bonds, whereas chelates are compounds containing a central atom bonded with surrounding ligands.
Below is a summary of the difference between ligand and chelate in tabular form.
Summary – Ligand vs Chelate
Ligand and chelate are related terms that are discussed mainly under the field of coordination chemistry. The key difference between ligand and chelate is that ligands are the chemical species that are donating or sharing their electrons with a central atom through coordination bonds, whereas chelates are compounds containing a central atom bonded with surrounding ligands.
Reference:
1. “Ligand.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.
Image Courtesy:
1. “Metal-EDTA” By Smokefootderivative work: Chamberlain2007 (talk) – Medta.png (Public Domain) via Commons Wikimedia