Ligands in coordination chemistry mean that they are molecules or ions that act as Lewis bases and donate their electron density to the vacant hybrid orbitals of the metal. Strong coordinate covalent bonds are formed with the metal. Ligands can be classified into various types on the basis of their denticity, charge, and field strength. They are crucial as they play an important role in determining the magnetic properties, geometry, and colour of coordination compounds. The word ‘ligand’ comes from Latin which means ‘to bind’. 

Types of Ligands

Ligands can be of two types based on the charge on them – neutral and anionic.

• Anionic ligands have a negative charge on them. Examples of atomic anions are halide ions, oxide, and nitride.
• Molecular anions are composed of more than one type of element. Hydroxide, cyanide, thiocyanide, nitrate, sulphate, acetate, etc, are common examples.
• Neutral ligands have a lone pair of electrons that they can donate to the metal. The most common examples are water, carboxyl, nitrosyl, etc.

Denticity and Hapticity

Denticity is defined as the number of donor sites present in the ligand. 

Ligands can be classified on the basis of their denticity as follows:

• Unidentate Ligand: If a ligand has only one donor site/group, it is called unidentate. Examples are Cl-, NH3, etc. 

• Multidentate Ligand: If a ligand has more than one donor site/group, it is called multidentate. Multidentate ligands show a chelate effect. It can further be divided into the following types based on the number of donor sites:

1. Bidentate Ligand: They have two donor sites, and both of them can participate in the bond formation simultaneously. Ethylenediamine is an example of a bidentate in which there are two nitrogen atoms that can donate electrons simultaneously to the metal centre. 
2. Tridentate and tetradentate ligands: They have three and four donor sites respectively. Diethylenetriamine is a tridentate ligand having three nitrogens that can donate electrons. Triethylenetetrammine is a tetradentate ligand having four nitrogens.
3. Hexadentate: There are six donor sites. Example: EDTA

• Polydentate Ligand: When a ligand can exhibit multiple denticities depending on the external conditions, it is called polydentate. EDTA (ethylenediaminetetraacetate) is an example that can donate a different number of electron pairs, ranging from one to six. It can bind with the metal centre via two nitrogens or via four acetate group’s oxygens. 

• Ambidentate: A ligand that has more than one donor site but only one of them can be involved in bonding at one time. For example, SCN- is ambidentate because it can donate from both sulphur and nitrogen centres. NO2- is ambidentate as it can donate from both oxygen and nitrogen centres.

Hapticity of a ligand is defined as the binding of the ligand with the metal centre through a series of contiguous atoms. Multiple atoms can coordinate with the metal via pi-bonding interactions. Ligands that exhibit hapticity are cyclopentadienyl anion, benzene, etc. It is represented by the Greek symbol 𝞰. The number of atoms coordinated with the metal at a given type is written at the top right corner of this symbol.

For example, if only two atoms of benzene are coordinated with the metal, then it would be called 𝞰2-benzene. If four atoms are coordinated, then 𝞰4-benzene.

Examples of such coordination compounds: ferrocene (iron is bonded with two cyclopentadienyl rings), nickelocene, etc.

Spectrochemical Series

Ligands can be classified into strong and weak field ligands on the basis of the extent to which they split the metal’s orbitals. A basic understanding of crystal field theory is essential for this. There are five d-orbitals in the metal that are degenerate, i.e., they are of the same energy. However, in the presence of a ligand, the orbitals can split into two energy levels. In an octahedral sphere, five d orbitals split into three and two d-orbitals. Three orbitals that are at a lower energy level are dxy, dyz, and dxz. Two orbitals that are at a higher energy level are dx2-y2 and dz2. These two orbitals experience more repulsion from the ligands as they are directed towards them in space. This causes them to rise higher in energy and form a set of orbitals.

The three orbitals dxy, dyz, and dxz lie between the axes and are lowered in energy. They form t2g set of orbitals.

This phenomenon is called crystal field splitting. Strong field ligands cause more splitting than weak field ligands. On this basis, a spectrochemical series has been formed in which ligands are positioned according to their field strength:

I- < Br- < Cl- < NO3- < F- < OH- < H2O < Pyridine < NH3 < NO2- < CN- < CO

Conclusion

Ligands are molecules or ions that bind covalently with the central metal ion and donate their electron density to their vacant orbitals. They can be classified into various types based on their denticities. They can be anionic or neutral. Ligands can split degenerate d-orbitals of metal into orbitals of different energy. Two d orbitals go to a higher energy level and the remaining three are at a lower energy level. This is termed as crystal field splitting. Now, let’s look at a few ligands in coordination compound questions.