Isomerism is the existence of multiple chemical compounds with the same chemical composition but different structures and properties. This phenomenon is not limited to organic compounds but finds itself in inorganic substances as well. Coordination complexes happen to be the substances in which various types of isomerism can occur.

The first type of isomerism to be discussed is coordination isomerism. It is one of the constituents of structural isomerism.

Structural Isomerism

Coordination Isomerism

Coordination isomerism involves different compounds with the same number of specific types of coordinating ligands, however, the ligands are linked to different metal ions. This may also be understood as the interchange of ligands linked with the cationic and the anionic complexes in a complex compound. In the example that follows, there are a total of two platinum ions, four ammonia ligands, and six chloride ligands in each of the coordination compounds:

[Pt(NH3)4][PtCl6] and [Pt(NH3)4Cl2][PtCl4].

Other pairs of coordination isomers can be:

[Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].

[Cr(NH3)6][Cr(C2O4)3] and [Cr(NH3)4C2O4][Cr(NH3)2(C2O4)2].

A brief description of other types of isomerism present in coordination compounds is given as follows:

Ionization Isomerism

Compounds that have the same chemical formula but form different ions in a solution show ionization isomerism. An example is that of the pair of complexes [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4. The two complexes essentially have the same empirical formula. Moreover, they have red and violet colors, and ionize to form Br and SO42- ions respectively, along with their corresponding cation.

Polymerization Isomerism

This type of isomerism occurs when the molecular formulae of the two compounds are integral multiples of their stoichiometric arrangement. For example, [Co(NH3)3(NO2)3] and [Co(NH3)6][Co(NO2)6]. The given pair complexes are an example of polymerization isomerism.

Hydrate Isomerism

Water is a relatively strong coordinating group and forms many aquo complexes. Further, the number of water molecules that are directly attached to the central metal atom/ion can vary. Several factors and conditions can cause this phenomenon. This variation in the number of water molecules present in the secondary or coordination sphere results in hydrate isomerism. An example of hydrate isomers is [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2.H2O, and [Cr(H2O)4Cl2]Cl.2H2O.

Linkage Isomerism

Linkage isomerism, as the name suggests, occurs when there is more than one atom in a ligand to link directly with the central metal atom/ion. An example is that of the NO2 group. When it acts as a ligand, it can be attached to the metal through either the nitrogen atom or the oxygen atom. It is called the nitro group in the former case, or nitrito group in the latter. 

The complex [Co(NH3)5NO2]Cl2 is named Nitropentaamminecobalt(III) chloride.

Alternatively, [Co(NH3)5ONO]Cl2 is called Nitritopentaamminecobalt(III) chloride.


Geometrical isomerism

Isomerism found in compounds having the same composition and molecular formula but having different relative positions of atoms or molecules in space is known as geometrical isomerism. Geometrical isomerism can also be called cis-trans isomerism. It is the most important type of isomerism found in coordination compounds and was predicted by Werner’s theory. 

The number of possible geometrical arrangements increases along with the coordination number.

Optical isomerism

Molecules having no symmetry (asymmetrical) rotate plane polarized light to either left (l – levorotatory) or right (d – dextrorotatory). Such molecules are called optically active. A mixture of d and l isomers is known as a racemic mixture, and the separation of the two isomers is called resolution.

Various coordination compounds can also exhibit optical isomerism by existing in the form of d- and l- isomers, due to asymmetry. The structures of these two isomers are mirror images of each other.

Optical and geometrical isomerism may collectively be called Stereoisomerism.

Concepts Berg

What is coordination isomerism with an example?

Coordination isomerism occurs when the ligands that are a part of the anionic and cationic complexes are interchanged with each other. [Co(NH3)6][Cr(C2O4)3] and [Co(NH3)4C2O4][Cr(NH3)2(C2O4)2] are coordinate isomers to each other.

What is isomerism in a coordination compound?

Isomerism is the phenomenon in which multiple molecules or compounds have the same chemical formula but differ in the arrangement of their structure. This causes the molecules to have different chemical and physical properties.

How do you name coordination isomers?

Coordination isomers are named like any other coordination compounds. This is according to the rules of nomenclature provided by the International Union of Pure and Applied Chemistry.

What are the types of isomerism in coordination compounds?

The types of isomerism that occur in inorganic coordination compounds are coordination isomerism, ionization isomerism, linkage isomerism, hydrate isomerism, linkage isomerism, optical isomerism, and geometrical isomerism.

What is geometrical isomerism in coordination compounds?

The isomerism that occurs when multiple compounds have the same molecular formula but differ in the spatial arrangement of atoms or groups of atoms is geometrical isomerism. It may also be called stereoisomerism or cis-trans isomerism. Geometrical isomerism happens to be of key importance in all of the types of isomerism present in inorganic complexes.

What is coordination position isomerism?

Coordination position isomerism is shown by polynuclear complexes when the relative positions of ligands are interchanged.

Which type of isomerism is present in octahedral complexes?

Octahedral complexes commonly possess stereoisomerism, which is divided into the geometrical (cis-trans/mer-fac) and optical (enantiomers) isomerism.