Ligands in Coordination Compounds

Coordination Compound

Coordination compounds are chemical compounds made up of an array of anions or neutral molecules bound together by coordinate covalent bonds to a central atom. Coordination complexes are another name for coordination compounds. The ligands are the molecules or ions that are bound to the central atom (also known as complexing agents).

Metal complexes are coordination compounds that have a metallic element as the central atom. The central atom in this type of coordination complex is usually a transition unit. It's worth noting that the coordination centre is the central atom in these complexes.

This article will study chelating ligands, types of ligands, and examples of chelating ligand in detail.


Properties of Coordination Compounds

  1. Since unpaired electrons absorb light in their electronic transitions, the coordination compounds produced by transition elements are coloured. Iron(II) complexes, for example, can be green or light green in colour, whereas coordination compounds containing iron(III) are brown or yellowish-brown in colour. 

  2. Due to the presence of unpaired electrons, the resulting coordination complexes have a magnetic nature when the coordination centre is a metal.

  3. The chemical reactivity of coordination compounds varies. They will take part in both inner-sphere and outer-sphere electron transfer reactions.

  4. Complex compounds with specific ligands have the ability to catalyse or stoichiometrically help in the transformation of molecules.


Ligands in Coordination Compounds

The ligands are the atoms, molecules, or ions that are bound to the coordination centre or the central atom/ion.

These ligands can be simple ions or molecules (like Cl- or NH3), or they can be relatively large molecules known as chelating ligands like ethane-1,2-diamine (NH2-CH2-CH2-NH2).


Types of Ligands

Based on the nature of the bond between the ligand and the central atom, ligands are classified as follows:

Anionic ligands: CN-, Br-, Cl-

Cationic ligands: NO+

Neutral ligands: CO, H2O, NH3


Unidentate Ligands 

Unidentate ligands are ligands that only have one atom that can bind to the coordination centre. A good example of an unidentate ligand is ammonia (NH3). Cl, and H2O.


Bidentate Ligands 

Bidentate ligands, such as ethane-1,2-diamine, have the ability to bind to the central atom through two separate donor atoms.

The ion oxalate is a bidentate since it can bind to the central atom of a coordination compound through two atoms. Ethane-1, 2-diamine:


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Chelate Ligand 

What is Chelating Ligand?

A chelate ligand is described as a polydentate ligand that binds to the same central metal atom through two or more donor atoms. The denticity of such ligands refers to the atoms that ligate to the metal ion.

Chelating ligands may be attached in a variety of ways. The Chelating rings often have a number of atoms. The metal ion is usually found with five or six atoms.


Example of Chelating Ligand:

Multidentate ligands are another name for chelating ligands. Chelates are the compounds that are produced by these compounds.


Types of Chelates:

Ethylenediamine is a common example of a chelating compound (NH2 CH2 CH2 NH2). With the two nitrogens present, it may form a bond with a metal ion. Oxalate and glycinate are two other examples of chelate complex..


Polydentate Ligands 

Many donor atoms in some ligands may bind to the coordination centre. Polydentate ligands are the name given to these ligands.

The EDTA4- ion (ethylene diamine tetraacetate ion) is a polydentate ligand with four oxygen atoms and two nitrogen atoms that can bind to the coordination centre. Given below is an example of chelating ligand:


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Ambidentate Ligands

Some ligands can bind to the central atom by using atoms from two separate elements.

The SCN- ion, for example, can bind to a ligand through the nitrogen or sulphur atoms. Ambidentate ligands are a type of ligand that has two ends.


Applications of Coordination Compounds

  1. Because of the colour of coordination compounds containing transition metals, they are widely used in industries for material coloration. In the dye and pigment industries, they are used.

  2. In the electroplating process, some complex compounds with cyanide as a ligand are used. These compounds are also beneficial in the field of photography.

  3. Many metals can be extracted from their ores with the aid of coordination complexes. Nickel and cobalt, for example, can be extracted from their ores using hydrometallurgical methods involving coordination compound ions.

  4. Haemoglobin is made up of a complex ion called Haeme, which has a tetrapyrrole Porphyrin ring structure with a central Fe2+ ion.

  5. Vitamin B12 has a coordination number of 6 and is made up of a tetrapyrrole porphyrin ring complex with a central Co+3 ion.

  6. The complexing agent Dimethylglyoxime is used to calculate Ni+2 (DMG). The hardness of water is determined by combining Ca++ and Mg++ complexes with EDA.

  7. Cisplatin is a chemotherapy drug that is used to treat cancer.

  8. In photography, film development is a complicated process.

  9. In metallurgy, the Mcarthur Forest Process uses a complex of cyanide ions to remove gold and silver.


Did You Know? 

Isomers are two or more compounds that have the same chemical formula but a different atom structure. Coordination compounds primarily show two forms of isomerism, namely stereo-isomerism and structural isomerism, as a result of this disparity in atom arrangement.

Stereoisomerism Stereoisomers are coordination compounds with the same chemical and chemical bonds in a different spatial structure. Optical isomerism and geometrical isomerism are two types of isomerism.

Optical Isomerism: Optical isomers, also known as enantiomers, are isomers that form non-superimposable mirror images. There are two kinds of these.

Dextro, also known as ‘d' or ‘+,' is the isomer that rotates plane-polarized light clockwise.

The levo isomer, also known as the ‘l', ‘-‘ isomer, rotates plane-polarized light in an anti-clockwise direction. The racemic mixture is the equimolar mixture of the ‘d' and ‘l' isomers.

Geometrical Isomerism: Owing to various potential geometric configurations of the ligands, geometrical isomerism is observed in heteroleptic complexes (complexes with more than one type of ligand). This behaviour is most common in coordination compounds with coordination numbers of 4 and 6.

FAQs (Frequently Asked Questions)

1. What is Chelate Compound? What is Chelating Ligand?

Answer: The hexadentate ligand ethylenediaminetetraacetic acid (EDTA) is another example of a polydentate ligand. The ring is formed as a result of this. Chelation is a chemical reaction in which a bidentate or polydentate ligand forms a ring around a metal atom. A chelate is the complex produced as a result of this process.

2. What Causes the Formation of Coordination Compounds?

Answer: The ability of ions and molecules to donate and accept electrons causes coordination compounds to form, resulting in the creation of additional bonds between them and hence increased stability. The greater the number of electrons involved in bond formation, the more stable the bond.

3. What are the Types of Ligands?

Answer: Ligands with just one donor atom, such as NH3, Cl-, F-, and others, are known as unidentate ligands.


Bidentate Ligands with two donor atoms, such as ethylenediamine, C2O42-(oxalate ion) etc. Tridentate ligands, such as (dien) diethyl triamine, have three donor atoms per ligand.

Hexadentate ligands, such as EDTA, contain six donor atoms per ligand.

Chelating Ligands: A chelating ligand is a multidentate ligand that coordinates to a metal ion via several sites at the same time. Chelate is a ring-like structure formed by these ligands. Chelation improves the complex's stability. The chelation effect is the name for this phenomenon.