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The transition metals or d-block elements have incomplete d-orbitals which can occupy electrons in the bond formation. Because of the presence of vacant d-orbitals, these elements have tendency to accept electrons from other atom/ion/molecule to form dative or coordination bond.

In such type of bonding, metal atom acts acceptor and the donor atom which is known as ligands, involve in bond formations. The bonding between acceptor metal atom/ion and donor ligand results the formation of coordination complex or coordination compound or coordination ion.

The coordination compounds have many unique features in bonding and have several industrial applications.

Coordination Compound

The coordination complexes are relatively un-reactive compounds compare to other compounds such as covalent or ionic compounds. Therefore they can isolate in solid or liquid forms. In their solutions also, they can ionize in two ions; counter ion and coordination ion. The bonding in coordination complexes is bit complicated and determine the stability the coordination compounds.

The number of coordination between ligands and metal atom/ion is called as coordination number. There are many ligands that can form more than one coordination bonds with similar or different atom with same metal atom/ion. In such case, the complex forms a cage like cyclic structure which is known as chelate.

What is Chelation?

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All the kinds of metals can involve in the complex formation up to some extent that may depend upon the electronic configuration of the metal. The charge on the complex can be positive or negative or zero that depends upon the kind of ligand and metal in the complex and form cationic or anionic or neutral complex.

The geometry of complex also varies with the coordination number of the complex. For example; coordination number with two forms linear complex, coordination number with 4 and 6 results the formation of tetrahedral and octahedral geometry respectively. For instance; the given complex of chromium has six coordination number and six ammonia ligands are arranged in octahedral manner around the metal ion that is Cr3+ion.

Octahedral Complex

On the basis of bonding with metal ion/atom, ligands can classify as monodentate, bidentate, tridentate or polydentate etc. Here the dent show the bonding while mono, di and tri represents the number of coordination bonds between metal and ligands. If a ligand forms more than one bond through different donor atoms, it is called ambidentateligand.

For example; -NO2ligand can bond through either nitrogen or oxygen to the metal atom/ion. If it bonds through nitrogen atom, it is named as nitro ligand while the bonding through oxygen atom makes nitrito ligand. The presence of such type of ligands in the molecule results the formation of hetero cyclic rings in the molecule that provides extra stability to the molecule. These rings are called as chelate rings and phenomenon is known as chelation.

The chelation definition states that the presence of more than two binding sites in the same ligand forms cyclic ring in the complex that is called as chelation. Usually bidentate, tridentate, tetradentate or polydentateligands have more than one potential binding sites that can be used by the ligand in the formation of chelate.

Lets take one example of bidentate ligand; named as ethylene diammine which forms a chelate with copper metal to form a chelate ring in which the nitrogen atoms of ethylenediammine are bonded with the copper. Some other examples are [PtCl3(CH2=CH2)]-, (benzene) tricarbonylchromium and ferrocene etc.

Chelation is a natural process to stabilized the complex and also involve in many biochemical such as involve in the prevention of absorbed nutrients like iron forming precipitation with phosphorus and enters in plant cells. There are many organic acids like citric acids, malonic acid, and some amino acids that have chelate rings in their molecular structure.

Chelating Agent

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Many of the chelate are involve in many biochemical reactions that play important roles in many biochemical processes such as oxidative phosphorylation, respiration and photosynthesis. The chelating agents act as catalyst and promoters for such reactions.

Similarly, they are used in several industrial processes for the manufacturing of organic and inorganic chemicals.

Let’s first discuss, what is a chelating agent?

Any molecule forming multiple bonds to a single metal atom /ion is called as a chelating agent. Such cyclic compounds are composed of a metal atom/ion and one chelating agent that is a poly-dentate ligand. There are many polydentate ligands such as ethylenediamine ( :NH2-CH2-CH2 -:NH2), ethylenetriammine etc.
Let’s discuss one chelate with any one of these chelating agent. One molecule of ethylenediamine contains two donor nitrogen atom in which each nitrogen atom has one lone pair of electrons which can involve in the coordination bonding with metal atom/ion.

Both nitrogen atoms of ethylenediammine molecule involve in coordination bond formation with the metal ion/atom like nickel (II) ion and can form five membered heterocyclic rings. Since Nickel (II) ion can form six coordination bonds therefore remaining four bonds can coordinate with any ligand such as water to form an octahedral coordination complex; [Ni(en)(H2O)4]2+. The abbreviation of NH2CH2CH2NH2 is written as 'en' or ethylenediamine.

If two molecules of ethylenediammine are bonded with the same metal ion than remaining two valences are fulfilled with water ligand to form [Ni(en)2(H2O)2]2+ while the presence of three ethylenediammine ligands results the formation of [Ni(en)3]2+ octahedral complex ion. Here, water is a monodentate ligand as it can form only one coordination bond through oxygen atom with the metal ion, therefore the presence of the water molecule cannot create any chelation in the molecule. The formation of the chelate provides the stability to molecule compare to non-chelate rings.

Another example of the chelating agent is porphine that can form multiple bonds to a metal ion through their nitrogen atoms (with lone pair of electrons). There are four nitrogen atoms in each porphine molecule which can form coordinate bonds with metal atom/ion to form the chelate ring. One of the best examples of porphyrine chelation is heme which acts as central component in the haemoglobin molecule.
We know that haemoglobin acts as oxygen carrier in the blood.

In this chelate ring, the porphyrin chelating agent is bonded with the iron (II) ion to form a stable cyclic ring. Same chelating agent is found in the chlorophyll molecule in which the cyclic ring is bonded with magnesium (II) ion through coordination linkage. Chlorophyll involves in the photosynthesis process and tends to absorb the visible range of light for the photosynthesis process.

Similarly, the porphyrin chelating agent is found in vitamin-B12 in which cobalt (II) ion is bonded with the chelating agent to form a stable complex. It is an essential component of living bodies but cannot synthesize in animals and plants. They are synthesized in certain bacteria and molds only. 2,3-dimercapto-1-propanol or dimercaprol is also a chelating agent which tends to form chelate complex with many heavy metals like gold, arsenic, antimony and mercury through di-sulphur linkage.

Chelating Ligand EDTA

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One of the most famous and useful chelating agent is EDTA that is widely used in a number of industrial processes. EDTA is an abbreviated form of ethylenediaminetetraacetic acid. The structural formula of EDTA is as given below.

EDTA- Ethylenediaminetetraacetate Ion
  1. In EDTA ion, there are total six donor atoms in the molecule; two nitrogen atoms and four oxygen (II) ions.
  2. Each nitrogen atom contains one lone pair of electrons which can donate to metal atom/ion to form coordinate bond.
  3. Similarly four oxygen ions carry negative charge and can form bond with the same or different metal atom/ion. It generally bonds with the transition-metal ions or main-group ions to form coordination complexes.
  4. This chelating agent shows wide applications in the manufacturing of soaps and detergents with the formation of coordination complexes with calcium or magnesium ions.
  5. EDTA complex ions are mainly used in the treatment of hard water which is mainly due to the excess of calcium and magnesium ions in the solution. The presence of these ions can interfere in the cleaning actions of soap and detergents.
  6. Therefore we need to remove them efficiently for better cleaning action.
  7. EDTA tends to form coordination complexes with these metals ions so can easily remove these ions from the water to make it soft in nature. The EDTA forms [Ca(EDTA)]2- with calcium ion in that complex EDTA acts as tetradentate ligand to form coordination bond through two nitrogen atoms and two oxygen atoms of carboxylate groups (-COO-).
  8. Like other chelating agents; EDTA also acts as stabilizing agent in the food industry and prevent the food spoilage that occurs mainly due to some naturally-occurring enzymes.
  9. These enzymes contain transition-metal ions that can form coordination complexes with EDTA and the chelation provides stability to the complexes therefore we can easily isolate them.
  10. The bonding of enzymes with EDTA removes the metal ion from the enzyme and deactivates it for further any biochemical reaction.
  11. The EDTA complexes also used to retain the color of packed food materials such as canned shrimps and clams, dried bananas, peas, beans, and pecan pie filling.
  12. The presence of EDTA complexes in food materials also improves the flavor of canned beverages, and sauces.
  13. It prevents the rancidity in packed salad, sauces, and other food materials.
  14. The metal salts of EDTA mainly dissolve the calcium carbonate scale that is deposited from hard water.
  15. Compare to the use of a corrosive acid, the use of EDTA complex is safe and effective.
  16. The rare earth elements are usually found in a mixture in their natural form and difficult to separate due to their similar chemical and physical properties.

But these metals form coordination complexes with EDTA which are varied in their stability. This variation helps in the separation of rare earth elements from each other and can be easily recover by simple physical methods.

The EDTA solution is also used as the anticoagulant in blood banks as it forms complex with calcium ions which are mainly involved in the coagulation process of the blood. The sodium salt of EDTA4- is used in the manufacturing of beer and mayonnaise also.

EDTA Chelation

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EDTA, ethylene diaminetetraacetic acid is a hexadentate ligand that acts as a chelating agent in the formation of coordination complexes. There are various applications of this chelating agent in biochemical, medical and other industries. The delivery of these coordination complexes through orally or intravenously is called as chelation therapy.

For example; many EDTA complex with sodium, calcium or magnesium metal ions are used for different purposes and that is known as edta chelation therapy. In this chelation therapy around 20-30 or more doses of EDTA complexes are delivered to the patient with vitamin/mineral supplements under the supervision of a doctor. It can be taken orally or by some other ways.

The infusion of these chelate complexes is used for changes in healthy lifestyle. The infusion of EDTA complex is medical treatment of lead poisoning and also for cardiovascular disease. Other benefits of EDTA infusion is in the hypertension, arthritis, stroke, cataracts, cardiac arrhythmia, Alzheimer’s and diabetes.

Chelation Benefits

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Let’s have a look on the benefits of chelating agents and other coordination complexes formed by these agents in various industries and areas. The chelating agents have ability to bind with iron in basic medium such as soil at high pH and transfer to the plants to increase the availability of nutrients for them.

Dimercaprol , a chelating agent is used for the treatment of Lewisite which is an arsenic-containing mustard gas and used in World War I. Once the chelating agent binds with the metal ion/atom, it cannot transfer from one cell to another and rapidly excreted from the body. Some complexes form an insoluble precipitate and prevent the mineral nutrients to leach with water in the soil.

The presence of chelating agents in the soil also prevents the unfavorable reactions of metal ions and makes them available for the plants. For example; iron (III) ion reacts with hydroxyl group in alkaline soil to form insoluble ferric hydroxide complex.

Some of the metal ions are quite toxic for the healthy development of plants, therefore the chelation reduces the concentration of metal ions in the soil. The chelation of metal ions present in the soil becomes more mobile and easy to uptake by plants.

Some of chelating agents tend to form complexes with iron and reduce the concentration of it in the soil which is a favorable condition for the development of plants but not for many plant pathogens. There are many naturally occurring chelating agents such as hydroxamate siderophores, organic acids and amino acids which are produced by plants or other microbes.

Out of these naturally occurring chelation complexes, hydroxamate siderophore is produced by soil microorganisms and helps in the movement of plants nutrients like iron. Chelates of amino acids such as Glycine and with organic acid like citric acid, gluconic acid and tartaric acid with transition metal ions like zinc, copper and iron are studied and use for several applications.

Chelation Therapy

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  1. The process of formation of complex molecules with the chelating agents and metal ions is called as chelation. For instance EDTA or ethylenediaminetetraacetic acid contains six donor atoms that can form coordinate bonds with metal atom/ion to form coordination complex.
  2. This chelating agent acts as antioxidant in fatty, oily foods and prevents the rancidity of the food materials.
  3. The use of chelating agent for the elimination of toxic metal ions from the system is known as chelation therapy.
  4. It is mainly used in medical industry for the removal of heavy toxic metals from organs such as EDTA chelating agent is used for the removal of lead and uses in the treatment of lead poisoning.
  5. Similarly excess of copper, zinc and iron can remove through complex formation and used in the treatment of Alzheimer's disease.
  6. The use of EDTA in this disease reduces the atherosclerotic plaques in blood vessels by chelation with calcium metal.
  7. The chelation therapy is useful in the treatment of many diseases yet it also consumes the calcium from bones and potentially dangerous for body.
  8. The accumulation of lead, arsenic, mercury and toxic metals in the body can affect the brain and nervous system.
  9. There are several hazardous effects of EDTA infusion such as excess amount of EDTA can deplete the concentration of metal ions and other essential nutrients in the body.
  10. Similarly presence of copper (II) ions and iron (II and III) ions can cause oxidative damage at high concentration.
  11. Therefore the excess of these metal ions can remove with the help of lipoic acid that act as chelating agent.
  12. Organic acids such as citric acid also acts as chelating agent and can bind with metal ions for making the water soft in soaps and detergents.

Iron Chelation

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The combination of small molecules with extra pairs of electrons and metal atom/ion results the formation of complex ion/molecule. These small molecules are known as chelators. Generally chelators are simple molecules such as ethylenediaminetetraacetic acid, ethylenediammine etc.

These chelators are usually chemically inert and used to detoxify metal ions. For example; the chelation of an iron chelator desferrioxamine helps in the removal of excess iron from the chronic blood transfusions.

Iron chelation is very useful in several industries and has medical value as well. The iron chelator can easily diffuse in the body with blood to target the metal ion. There are several iron chelators which can be classified on the basis of their origin, or interaction with solvents and stoichiometric interaction etc.

Almost all the living bodies survive on iron as this metal plays a vital role in red blood cells as well as in the chlorophyll in leaves which involves in the photosynthesis process to produce sugar molecules. Therefore it is essential to intake this metal in our diet or by iron supplements for good health.

Since iron is a transition metal therefore contains incomplete d-orbitals and can accept extra pairs of electrons to form coordination bond with ligands to form coordination complexes. The chelated iron can easily absorb by plant and animal bodies as the cell membranes allow these chelate to pass through them.