Poly dentate ligands are the type of ligands that can ring around the metal center and are often described as chelating agents. The word chelate comes from the Greek language meaning crab-claw or bite. The name indicates these ligands have two or more donor atoms and grab the central metal like a crab can grab something. Due to these two or more donor atoms, these ligands form coordination bonds with a transition metal.

For example, Ethylenediamine (en) is a bidentate ligand that contains two donor atoms. It coordinates with the central transitional metal in a complex formation.

Background and Discovery

In 1869, a scientist Blomstrand propose a chain structure of hexamine cobalt(ii) chloride. In this structure, ammonia molecules were linked together to form a chain.  from the discovery of the ring-like concept was use then used by Werner in 1893 to give his revolutionary structure of ethylenediamine(en)Pt(II) Cl2 can be formed from Pt(II)NH3Cl2 by replacing four ammonia (NH3) molecules with two ethylenediamine. So he concluded the “ethylenediamine” bound with Pt at two ends through the phenomenon of chelation.


The IUPAC set up some rules for the naming of metal complexes. While naming the metal chelates the symbol of metal cation should be written first with its primary valency and then formulas of anionic ligands are placed. In chelates, the binding atom can be properly mentioned, for example,  1,1′ dithiooxalto.

Conditions necessary for chelation

The conditions which are necessary for a ligand to act as a chelating agent are given below;

1. Appropriate donor atoms

The ligand should have more than one donor atoms or group which hare capable of combining with metal cation by a coordinate bond. These donor atoms or groups may be acids or base or their derivatives. For example, -COOH, SO3H, -OH, and -NH2.

2. Capable of ring formation

These functional groups discussed above must be situated in the molecule so that allow ring-like structure by overcoming the steric factors. For example, Ethylenediamine

Stability of chelates

The formation of a complex is taken place in a reversible reaction. The thermodynamic stability is the scale to study the comparative stability of chelates with normal complexes.

Factors influencing the stability of metal chelates

There are many factors that may influence the stability of metal chelates. Their efficiency and effectiveness are keenly considered with the targeted metals. Some factors are described as under:

  • The size of the chelating agent and the chelate ring
  • The possible no fo rings formed during chelation. For example, such chelating agents which formed rings are relatively more stable
  • Steric effects
  • The denticity is the nature of ligand groups

Classification of chelating agents

The detailed classification of the chelating agent is based upon the nature of their functional group attached. The main classes are as follows:

  1. Basic chelating groups amine
  2.  Acidic chelating groups. E.g, R-COOH
  3. Two acidic chelating groups. E.g.  oxalic acid
  4. Two basic chelating groups ethylenediamine
  5. One acid and base chelating group. E.g., glycine

Structure and Geometries

The structures of metal chelate are so much similar to metal complexes. according to crystal field theory During elucidation of molecular geometry. The electron geometry is much more important to describe clearly. This is because the stability of a complex depends on the arrangement of ligands as point charges. 

Types of geometries of metal chelates

To develop an understanding of the geometries of chelating is really important to track the application of these substances. Some important geometries are given discussed below;

  1. Octahedral geometries

Octahydra geometries show optical isomerism which is also evidence of chelation. For example, Fe3+ is chelated when treated with ethylenediamine and shows mirror images shown it is also known as R/S configuration explain in the figure given below:

There are two main principal methods employed to study and investigate the structure of metal chelates.

Example of chelating agents


It is the most commonly used chelating agent. It is a polydentate ligand with the ability to form six-coordinate bonds. For example, In an anionic complex of cobalt, it acts as a hexadentate ligand. six possible donor atoms are available in EDTA, therefore it has prime importance in complexometry.

Chelating agent EDTA

In lab analysis, versene 220TM is the crystal of di sodium salt of EDTA is used in water analysis such as the hardness of the water.

Medicinal application of chelating agents

zinc chelator in medicine therapy

TPEN (N,N,N′,N′-tetrakis(2-pyridinylmethyl)-1,2-ethylenediamine) has a great affinity for zinc. It is extensively used in medical therapy.

oral iron chelators

 Deferasirox is formed when first treated with iron salts to form stable iron chelates. It is an effective therapy for the treatment of cardiac siderosis. It is taken thrice a day with other supplements to improve the quality of life of patients.

Lead chelators

(Dextro)-Penicillamine is a chelating agent used in heavy poisoning treatments. It is also used in the treatment of kidney stones. It is an oral drug.

chelating agent in cosmetics

They are also used in soaps to capture heavy metals to avoid rancidity and discoloration. In creams and skin care products, It is incorporated to improve the shelf life and efficacy of vitamins, essential oils, and fats.

Natural chelating agents are added to enhance the shelf life of cosmetic products. Because their biodegradable property minimizes the environmental risk. When they are used in combination with antioxidants, such as tocopherol, natural chelating agents help to protect valuable ingredients like unsaturated oils. Common chelating agents used in cosmetics are given below;

  • Ethylenediaminetetraacetic acid (EDTA) 
  • Etidronic acid
  • Galactaric acid,
  • Sodium metasilicate 
  • Phosphate derivatives


Chelating agents

Concepts Berg

What Does Chelating Agent Mean?

Chelating agents are the multidentate ligands that have the tendency to bind with transition metal centers through more than one potential donor atom.

What is a natural chelating agent?

Amino acids are the most commonly present chelating agents in plants and animals. For example, in living organisms, glycine with both positive and negative charges present is bound with minerals. Fluvic acid is one of the most powerful carbon-containing chelating agents present in plants. 

What is the most common chelating agent?

The commonly used chelating agents are amino acids, oxaloto(ox), actylacetato(acac), Dimethlylglyoxime(DMG) and Ethlylenediaminetetraacetate(EDTA).

What are chelating ingredients?

The chemical which is used as chelating agents in cosmetics such as shampoos and beauty creams are di-succinate, Sodium citrate, disodium EDTA, and potassium citrate.

What are the structural characteristics of a chelation agent?

The chelating agents have a characteristic bi-tooth structure to claw or bind with the central transitional metal atom.

How is citric acid used as a chelating agent?

Citric acid is used in soil remediation to chelate the excessive iron present in the soil. Thus it controls the precipitation of Iron hydroxide and disulfide.

What is chelation and chelate effect?

The chelation or chelate effect is the ring-like structure formation between the metal ion and polydentate ligand to form a thermodynamically stable compound.

What is a chelate ligand?

The ligand with more than one donor atom can form a ring-like structure by binding with the central metal through multi linkage. 

What are the factors which affect the stability of chelates?

The factor which affects the stability

What is the difference between the chelate ligand and ambidentate ligand?

Chelate ligand has two or more donor atoms to bind with the metal in a complex simultaneously whereas, ambidentate somehow has two donor atoms but it can only donate one lone pair to form a coordinate at a time.

Why are chelating ligands more stable than monodentate ligands?

The chelate formation causes the symmetrical distribution of bonded pairs of electrons, which leads to thermodynamic stability.

What is the difference between complex and chelate?

The complex is the chemical compound that contains a central metal atom and any time of ligand to form a coordinate bond. In chelates, On the other hand, the ring-like structure of the coordination complex is formed by the multi-dentate bond of ligands.

How does EDTA chelate calcium?

Calcium forms a stable hexadentate with EDTA at pH 12.

When to use a chelating agent in your DIY formula?

Metal ions can cause rancidity and degradation of colors and vitamins so they are better to chelate in the DIY formula.

Reference Books

  • Chelating Agents and Metal Chelates by F Dwyer