Ligands are atoms, molecules, or ions that have non-bonded (lone pairs) electrons attached to the central transitional metal atom through coordinate bonds to form a coordination complex. They are classified on the basis of their tendency to make coordinate bonds. This tendency of ligands is known as their denticity. There are different types of ligands with different denticities used in coordination chemistry.
Denticity comes from the Greek word “dentis” meaning tooth. So, the number of bites/bonds from the ligand to the central atom is known as its denticity.
Some major types of ligands are monodentate ligands, bidentate ligands, bridging ligands, polydentate ligands (chelate ligands), etc.
1. Monodentate ligands
Monodentate ligands are the type of ligands that have the ability to share one lone pair of their donor atom to a central electron-deficient atom in coordination compounds. As the name indicates ‘mono’ means one and ‘dentate’ means tooth. This suggests that monodentate ligands bind to metal ions with one lone pair.
They are further classified into the following categories:
The monodentate ligands that bear negative charges are negative ligands. These are given below:
These ligands have one lone pair for coordination and bear no charge. Some examples are mentioned below:
Special monodentate ligands
Some ligands like carbonate CO3 2-, NO2–, NH2-NH2, etc have two donor atoms but due to strain in the complex, they only can donate one pair of electrons. Thus, they act as monodentate. These ligands are also named pro ligands.
This class of monodentate ligands has two donor atoms but only one of them at a time is able to bond with the central metal atom. For example, NO2– and SCN–.
2. Polydentate ligands (Chelate ligands)
Ligands that can bind with central metal atoms with more than one donor atom are called multidentate or polydentate ligands. They are also referred to as chelate forming ligands. The word chelate is derived from the word “chele” meaning claw. They bind with central metal atoms to form a closed ring-like structure. Chelates are relatively more stable than simple complexes due to the chelating effect.
They are classified further in these types as explained below:
Bidentate ligands have 2 donor atoms such that they can bind with the same central metal atom through two different donor atoms. Some common examples are given below:
- Acetylacetone (AcAc)
- Dimethylglyoxime (DMG)
- 8-hydroxyl quinoline
- Oxalato (ox)
Symmetrical and unsymmetrical ligands
They are bidentate ligands. However, if both the donor atoms are the same, they are called symmetrical bidentate ligands. If the donor atoms are different, they are known as unsymmetrical bidentate ligands. For example,
Symmetrical bidentate ligands
- Ethylenediamine (en)
Unsymmetrical bidentate ligands
- alpha nitroso beta naphthol
These ligands have three donor atoms.
- diethyl triamine (dien)
- 2,2’,2” terpyridine
Following the order made by the above ‘bidentate’ and ‘tridentate’ ligands, tetradentate ligands have a denticity of four.
Examples of tetradentate ligands:
Ethylenediaminetetraacetate (EDTA) is the ligand with six donor atoms. It has an application in water analysis.
3. Bridging ligands
Bridging ligands are ligands having lone pairs that connect two or more atoms. They act as a bridge or glue to hold two or more atoms together.
Mechanism of bridging by pyrazine ligand
4. Cyclic or ring ligands
Cyclic or ring ligands are special types of ligands that have cyclic structures. They have several lone pairs available for donation. When chelate is formed with metal atoms, it is very stable as compared with noncyclic complexes. For example, porphin, vitamin B12, hemoglobin, and chlorophyll form a ring around the metal atom in the coordination complexes.
One such example, ‘Prophin’ is given below.
Classification based upon nature of ligands
According to the Crystal Field Theory (CFT), ligands are the point charges. When ligands approach the central transitional atom they are repelled by the already existing electron pairs. Due to this repulsion, degenerated orbitals are split into two groups. Practically, it is difficult to quantitively calculate the crystal field splitting energy (CFSE) by the CFT approach. Hence, CFT is modified to ligand field theory.
Ligand field theory (LFT) describes the electronic structure of coordinated complex compounds. Ligands are divided into two main categories based on their field. There are several factors that affect this crystal field splitting.
Strong field ligands
Strong field ligands cause great repulsion when approaching the central metal atom. Hence, the electron in d orbitals of the metal atom starts pairing up. They are good Lewis bases. Their splitting increases the pairing energy in strong field ligands.
𝚫0 > P
Examples of some strong ligands or strong field ligands:
- CO, etc
Weak field ligands
Weak field ligands cause less repulsion to the electronic structure of metal atoms. The coordination compounds thus formed are usually paramagnetic in nature and have low wavelength colors. In case of weak ligands splitting energy is less than pairing energy. Hence, these ligands are also known as low spin ligands. They are weak Lewis bases.
Their splitting is less than the pairing energy.
𝚫0 < P
Examples of some weak ligands or weak field ligands:
- OH–, etc
Some ligands are intermediate in the ability to split the d orbitals. For example, ethylenediamine (en), H2O, NH3, F–, etc.
Summarizing the types of ligands
These elements are arranged in increasing order of their field effect in a complex. This arrangement is called the electrochemical series.
I < Br < Cl < F < OH < OH2 < NH3 < en < CN < CO
The table given below gives a general perspective of the types of ligands.
What are chelates? Which type of ligands forms chelates?
Chelate are coordination complexes in which a central metal atom is attached with ligands with more than one of its donor atoms. As its name suggests these ligands make a claw by binding up with the central atom in more than one place.
What does an unsymmetrical ligand mean?
Unsymmetrical ligands are bidentate ligands that have two donor atoms but these atoms are different from each other.
Why is OH– a better ligand than O2?
The reason is that; OH– is the type of ligand that bonds with the central metal atom through sigma donation whereas, O2 binds up with weak pi bonding.
What are strong and weak field ligands?
Strong field ligands are those which have high charge density, e.g NH3 whereas, weak field ligands have low charge density e.g. water.
Is EDTA a negative or a neutral ligand?
EDTA is a negatively charged ligand with a (-4) charge. It has six donor atoms and donates six electron pairs to form a coordination complex. For example, Co[EDTA]–, etc.
What is the water of crystallization? Is it the ligand water or the water of hydration?
The process in which water molecules become part of the crystal in a stoichiometric ratio is called water of crystallization or water of hydration. For example, CuSO4.5H2O. While on the other hand, water ligands are bonded with central transitional metal through coordinate covalent bonds to form complexes. For example, Cu[H2O]6+2, etc.
- Chemistry (IB Diploma Programme): Second edition By Christopher Talbot, Richard Hardwood, and Christopher Coates
- Coordination Chemistry By Fred Basolo and Ronald C. Johnson
- Ligands – Meaning, Overview, Explanation, and Examples (embibe.com)
- Ligands and Types (vedantu.com)