Chelation is a very common term used in different branches of science like chemistry, biology, and medical sciences. The process of chelation is widely used in the detoxification of the toxicants and in making complexes. Let us come to our main question, what is chelation?
Let us discuss the chelate meaning, a chelate is a compound that has two or more coordinate or dative bonds between a ligand (usually organic) and a central metal atom.
Chelation definition- Chelation is a phenomenon or ability of ions and molecules to form bonds with metal ions. Between a polydentate ligand and a single central atom, two or more different coordinate bonds are formed or present. Let us discuss the terms used in the chelation definition for a better understanding of the definition.
Ligand - A ligand is an ion or molecule that forms a coordination complex by donating a pair of electrons to the central metal atom or ion.
Polydentate - The number of atoms used to bind to a central metal atom or ion varies among polydentate ligands. Hexadentate ligands, such as EDTA, have six donor atoms with electron pairs that can bind to a central metal atom or ion.
Now, Let Us Discuss What is Chelation in Chemistry?
The chelate effect is when chelating ligands have a higher affinity for a metal ion than equivalent non-chelating (monodentate) ligands. Let's take a look at how the chelation mechanism works. The chelate effect is supported by certain thermodynamic concepts. Let's take a look at an example: Copper(II) affinities for ethylenediamine (en) and methylamine are compared.
1. Cu2+ + en ⇌ [Cu(en)]2+
2. Cu2+ + 2 MeNH2 ⇌ [Cu(MeNH2)2]2+
The copper ion forms a chelate complex with ethylenediamine in the first equation. Chelation results in the creation of a CuC2N2 chelate ring with five members. The bidentate ligand is substituted by two monodentate methylamine ligands with roughly the same donor strength in the second reaction, suggesting that the Cu–N bonds are similar in both reactions.
The equilibrium constant for the reaction is taken into account in the thermodynamic approach in explaining the chelate effect. The higher the equilibrium constant, the higher the complex concentration.
[Cu(en)] = [Cu][en]
[Cu(MeNH2)2] = [Cu][MeNH2]2
For the sake of clarity, electrical charges have been removed. The subscripts to the stability constants, show the stoichiometry of the complex, and the square brackets indicate concentration. The concentration [Cu(en)] is much higher than the concentration [Cu(MeNH2)2] because the analytical concentration of methylamine is double that of ethylenediamine and the concentration of copper is the same in both reactions.
As we know, ΔG = ΔH - TΔS
The discrepancy between the two stability constants is due to the effects of entropy since the enthalpy should be about the same for the two reactions. There are two particles on the left and one on the right in equation one, while there are three particles on the left and one on the right in equation two.
This distinction suggests that when a chelate complex is formed with a bidentate ligand, less entropy of disorder is lost than when a complex is formed with monodentate ligands. One of the variables that contribute to the entropy gap is this. Solvation shifts and chelate ring formation are two other factors to consider.
The enthalpy changes for the two reactions are nearly equal, indicating that the entropy expression, which is much less unfavourable, is the key reason for the chelate complex's greater stability. It's difficult to account for thermodynamic values in terms of changes in solution at the molecular level exactly, but it is apparent that the chelate effect is primarily an entropy effect.
The ligands (electron donors) used in the chelation process are known as chelants, chelators, chelating agents, and sequestering agents. These molecules are generally organic compounds, but this is not a necessity. As there are some cases of zinc and other inorganic molecules that are used as chelants.
Some of the examples of chelates are given below:
Chelate Compound Uses
Chelation is useful in providing nutritional supplements.
It is used in chelation therapy to remove toxic metals from the body.
Chelate compounds are used as contrast agents in MRI scanning.
These compounds are used in the manufacturing of homogeneous catalysts.
It is used in chemical water treatment to assist the removal of metals and in fertilizers.
The chelation process is used by plants for the removal of heavy metals.
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