Ligands

A ligand is a chemical term that refers to atoms, molecules, and ions that can be bonded to a central atom (metal or metalloid). In general, a ligand will provide at least one electron when participating in a bond, acting as a Lewis base. But in a few cases, the ligand accepts electrons and acts as a Lewis acid. In organic chemistry, ligands are often used to protect other functional groups (eg, ligand BH₃ protects PH₃) or to stabilize some readily reactive compounds (eg, tetrahydrofuran is a ligand for BH₃). A compound in which a central atom and a ligand are combined is referred to as a complex. An atom in a ligand provides a lone pair of electrons that directly forms a coordinate bond with a central atom, and such atom is called a coordinating atom, such as C in CO. The outermost electron layers of the coordinating atoms have lone pairs of electrons, and common coordinating atoms are non-metallic elements with relatively high electronegativity, such as N, O, C, S, and halogen.

Figure 1. Monodentate ligand

Figure 2. Multidentate ligand

Applications

Ligands have a wide range of applications in analytical chemistry, industrial production, and biochemistry.

• Analytical chemistry

Ligand can be used as an analytical reagent. In analytical chemistry, a ligand can be used as an analytical reagent for an element to determine the elemental content of a substance containing the ligand element. Ligands can be used to separate ions. Ligands can also be separated from other ions by forming complexes that alter the solubility of the species. For example, ammonia water reacts with AgCl, Hg₂Cl₂, and PbCl₂ to separate the cations. Ligands can block the interfering ions.

• Industrial production

Ligand can be used as a synthetic catalyst. In the pharmaceutical industry and industrial production, many catalysts must be used in the synthesis process. It is thus used in the preparation of pharmaceuticals and the production of chemical products.Some ligands can be used as mirror materials. For example, an amino ligand can be used to synthesize a coordination compound silver ammonia solution. And the silver mirror reaction can be used to coat a bright silver coating on the back of the glass.

Ligand can extract metals. The ligand can be purified by mixing with a metal to achieve purification of the mixed metal. For example, in the process of gold extraction by cyanidation, since a stable complex ion [Au(CN)₂] is formed, inactive gold can be introduced into the solution to perform gold extraction.

Ligand can be used as a material precursor. Ligand can be used as a material precursor for the synthesis of alumina particles and gallium arsenide (GaAs) films.

Ligands can soften the hard water. Hard water contains many metal ions, and the ligand can be precipitated by forming a coordination compound with metal ions, thereby achieving softening of hard water.

• Biochemistry

Ligands can be used in the synthesis of biomolecules. Many biomolecules are complexes. The normal functioning of many enzymes and magnesium-containing chlorophyll is also inseparable from the complex mechanism of ligands. Therefore, ligands can be used as raw materials for biomolecular synthesis.

Ligands can be used for detoxification of heavy metals. Detoxification agents such as ethylenediaminetetraacetic acid and 2,3-dimercaptosuccinic acid can be used for the detoxification of heavy metals, often because these compounds contain ligands that can be combined with heavy metal ions, and the resulting complex is less toxic, thereby achieving the purpose of detoxification.

Classification

Typical ligands can be classified according to their charge, size, their atomic properties, and the number of electrons that can be supplied. The size of the ligand can be expressed by its cone angle. Typically, the ligands are divided into monodentate ligands and multidentate ligands, depending on the number of coordinating atoms in the ligand.

• Monodentate ligand: There is only one coordinating atom in the monodentate ligand, such as NH₃, H₂O and so on. Some ligands contain two pairs of lone pairs of electrons, but since the two coordinating atoms are too close together, each ligand can only select one coordinating atom to form a bond with the central atom. Such ligands are still monodentate ligands.
• Multidentate ligand: The multidentete ligand contains two or more coordinating atoms, such as diethylene triamine H₂NCH₂CH₂NHCH₂CH₂NH₂ (abbreviated as DEN) and ethylenediaminetetraacetate (abbreviated as EDTA).

References

1. Zhang, Xi-Chang. (2016). “A supramolecularly tunable chiralDiphosphine ligand: Application to Rh and Ir-catalyzed enantioselective hydrogenation.” Chemical Science 7(7), 4594-4599.
2. Lentz. (2018). “Mallet-Ladeira. Germylene-sulfoxide as a potential Hemilabile Ligand application in coordination chemistry.” Dalton Transactions 7(44), 15751-15756.
3. Biswas. (2017). “Synthesis and characterization of a ruthenium complex with bis(diphenylphosphino) propane and thioether containing ONSdonor ligand Applic- ation in transfer hydrogenation of ketones.” Polyhedron. 131(7), 5277-5387.