Coordination Compounds

Isomerism in Coordination Compounds

Certain compounds such as coordinates have an identical formula but different properties and different structure. Even though they have the same formula they carry different color. You might be wondering which are these compounds. To know more about isomerism in coordination compounds, let’s dig deep into this interesting topic in the section below.

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Isomerism in Coordination Compounds

What are Ligands?

A metal ion in a solution cannot exist in isolation but in combination with ligands such as solvent molecules or simple ions that form complex ions or coordination compounds. These complexes have a central atom or ion which is often a transition metal.

These complexes also have a cluster of ion or neutral molecules surrounding ligands that bond with a coordinated bond to a central metal atom or ion. Ligands act as Lewis bases (electron pair donor) or as Lewis acids (electron pair acceptor).

Complex Ions

The ability of metallic elements to act as Lewis acids that form metal complexes with a variety of Lewis bases is their most important property. A metal complex has a central metal atom or ion that bonds to one or more ligands. These are the complex ions. A coordinate compound contains one or more metal complexes.

Why are Coordination Compounds Important?

Mentioned below are three reasons explaining why coordination compounds are essential.

  • The periodic table has most of the elements that are metals, and almost all metals form complexes.
  • Many industrial catalysts are metal complexes and important to control reactions.
  • Transition metal complexes are essential in biochemistry.

You can download Coordination Compounds Cheat Sheet by clicking on the download button below
Geometric and Optical Isomerism


Isomers are two or more different compounds having the same formula. There are two principal types of isomerism in coordination compounds:

  1. Stereo isomerism in coordination compounds
  2. Structural isomerism in coordination compounds

Coordination compounds

What are Stereoisomers?

Stereoisomers are the isomers that have the same atoms, the same set of bonds that differ only in the relative orientation of these bonds. These are subdivided as follows:

  • Geometrical isomerism: Geometrical isomers are possible for square, planar, and octahedral complexes, but not tetrahedral.
  • Optical isomerism: This isomerism is possible for both tetrahedral and octahedral complexes, but not for square and planar.

Isomerism in Coordination Compounds

                                                                                                                                                          Source: Wikipedia

What are Structural Isomers?

Structural isomers are the isomers that have the same molecular formula, that differ in the bonding patterns and atomic organization. Structural isomerism is subdivided as below:Isomerism in Coordination Compounds                                                                                                                                             Source: Wikipedia

Coordination Isomerism

Here the compounds contain complex anionic and cationic parts occurring by the interchange of some ligands from the cationic part to the anionic part. For example,

[Co(NH3)6] [Cr(C2O4)3]    isomer 1
[Co(C2O4)3] [Cr(NH3)6]    isomer 2

Ionization Isomerism

Here, isomers occur because of the formation of different ions in a solution. For example,

[PtBr(NH3)3]O2NO2      anion in solution
[Pt(NO2)(NH3)]Br          anion in solution

However, both anions are necessary to balance the charge of the complex. Moreover, the difference in both isomers is one ion directly attaches to the central metal but the other does not.

Hydrate Isomerism

A well-known and the best example of this isomer is chromium chloride, Cr Cl3.6H2O, which may contain 4, 5, or 6 co-ordinate water molecules.

[CrCl2(H2O)4]Cl.H2O     bright green
CrCl(H2O)5]Cl2.H2O      grey-green
Cr(H2O)6]Cl3                   violet

Solved Question For You

Question: What are the applications of coordination compounds?

Ans: Coordination compounds have the following applications.

  • Dyes and pigments: Ancient Greeks used a red madder dye for various purposes. A recent example is copper phthalocyanine, which is blue.
  • Analytical chemistry: Some coordination compounds are used as reagents in the laboratory for color tests, gravimetric analysis and complexometric titration and masking agents.
  • Sequestering agents: We can get rid of objectionable ions in industrial processes with coordination compounds.
  • Extraction of metals: Leaching of metals is done from their ores by the formation of stable complexes. For example, Ag and An as complexes of cyanide ions.
  • Bio-inorganic chemistry: Naturally occurring complexes are hemoglobin, chlorophyll, vitamin B12, etc.
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