The Types and Applications of Ligands


Posted November 25, 2020 by Bonnibelle

Ligand is a chemical term that refers to atoms, molecules, and ions that can bond with the central atom (metal or metalloid). The ligand will provide at least one electron when participating in bonding.

 
The definition
Ligand is a chemical term that refers to atoms, molecules, and ions that can bond with the central atom (metal or metalloid). The ligand will provide at least one electron when participating in bonding. The ligand plays the role of 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 (for example, ligand BH3 can protect PH3) or to stabilize some easily reacted compounds (for example, tetrahydrofuran is used as a ligand for BH3). The compound composed of the central atom and the ligand is called a complex.

Metals and metalloids can only exist in a gaseous state without being bonded to other atoms in a highly vacuum environment. In addition, metals and metalloids will bond with other atoms by coordination or covalent bonding. The ligand in the complex dominates the activity of the central metal, and the activity of the central metal is also affected by the speed at which the ligand itself is replaced and the activity of the ligand. In the fields of bioinorganic chemistry, medicinal chemistry, homogeneous catalysis and environmental chemistry, how to choose ligands is an important topic.

Types of ligands

Generally, ligands can be classified according to their charge, size, their atomic properties, and the number of electrons that can be provided (such as tooth occlusion or Hapto number). The size of the ligand can be expressed by its cone angle.

According to the number of coordinating atoms in the ligand, the ligands can be divided into monodentate ligands and multidentate ligands.

Monodentate ligand: A ligand with only one coordinating atom in a ligand. Such as NH3, H20, etc.

Multidentate ligand: A ligand with two or more coordinating atoms in a ligand. Such as diethylene triamine H2NCH2CH2NHCH2CH2NH2 (abbreviated as DEN) and ethylenediaminetetraacetate (abbreviated as EDTA).

Two ligands: Although some ligands contain two lone pairs of electrons, because the two coordinating atoms are too close, each ligand can only choose one coordinating atom to form a coordination bond with a central atom. Ligands are called two ligands. The two ligands are still monodentate ligands.

Coordination compound is a kind of compound with a characteristic chemical structure. It consists of a central atom (or ion, collectively referred to as a central atom) and the surrounding molecules or ions (called ligands/ligands) which are completely or partially combined by coordination bonds.

It contains complex molecules or ions formed by the combination of a central atom or ion and several ligand molecules or ions through coordination bonds, usually called coordination units. All compounds containing coordination units are called coordination compounds. The branch of chemistry that studies complexes is called coordination chemistry.

Complexes are a larger subcategory of compounds, which are widely used in daily life, industrial production and life sciences, and their development has been particularly rapid in recent years. It is not only related to inorganic compounds and organometallic compounds, but also has great overlap with the current frontiers of chemistry, cluster chemistry, coordination catalysis and molecular biology.

Applications of coordination compounds

In analytical chemistry, complexes can be used for:

Ion separation: change the solubility of the substance by generating complexes, thereby separating it from other ions. For example, the reaction of ammonia water with AgCl, Hg2Cl2 and PbCl2 to separate the first group of cations:

And use the formation of ammonia complex to get Zn into solution:

Titration of metal ions: For example, when quantitatively determining the content of Fe in the solution, the indicator is dark red [Fe(phen)3].

Masking interfering ions: Use the generated complex to eliminate the factors that will interfere with the results in the analysis experiment. The colorimetric determination of Co will be interfered by Fe. F and Fe can be added to generate a colorless stable complex ion [FeF6] to mask Fe:

In industrial production:

Coordination catalysis: The mechanism of catalytic reaction often involves coordination compound intermediates. For example, in the synthetic ammonia industry, copper acetate diammine is used to remove carbon monoxide, organometallic catalysts catalyze the polymerization of olefins or oligomerization catalysis, and asymmetric catalysis in drugs preparation.

Mirror making: using silver ammonia solution as raw material, using silver mirror reaction to plate a bright silver coating behind the glass.

Extraction of metals: For example, in the step of extracting gold by cyanidation, due to the formation of stable complex ions , inactive gold enters the solution:

The thermal decomposition of many carbonyl complexes can also be used to purify metals. For example, in the Mond method, the purification of nickel utilizes the reversible reaction of the formation and decomposition of nickel tetracarbonyl:

Material precursors: synthesis of alumina particles and gallium arsenide (GaAs) thin films.

Hard water softening

In biology, many biomolecules are complexes, and the combination of iron-containing hemoglobin with oxygen and carbon monoxide, the normal operation of many enzymes and magnesium-containing chlorophyll are also inseparable from the complex mechanism. The commonly used cancer treatment drug cisplatin, namely cis-[PtCl2(NH3)2], can inhibit the DNA replication process of cancer cells, and contains a flat square complex configuration. Antidotes such as ethylenediaminetetraacetic acid, sodium citrate, and 2,3-dimercaptosuccinic acid can be used in the detoxification mechanism of heavy metals. Often they can be combined with heavy metal ions to convert them into coordination compounds with little toxicity. So as to achieve the purpose of detoxification.

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Last Updated November 25, 2020