Isomerism in Coordination Compounds: Organic compound pentane \(\left( {{{\rm{C}}_5}{{\rm{H}}_{10}}} \right)\) can exist in three forms, namely \({\rm{n}}\)-pentane, iso-pentane and neopentane. What is the similarity and difference between them? They have the same molecular formula but differ in a structural arrangement.

There are several compounds that exist with the same chemical composition. These compounds exist as coordinates having different color and different structural formula. Such compounds are called Isomers. In this article, you will study different types of isomerism in coordination compounds.

Coordination Compound

Coordination compounds are the compounds in which the central metal atom is linked to several ions or neutral molecules by coordinate bonds, i.e., by donation of lone pairs of electrons by these ions or neutral molecules to the central metal atom.

The electrically charged species formed by the union of central metal atoms or ions with one or more ligands are called complex ions. The molecular or ionic species which gets directly attached to the central metal atom or ion during the formation of a complex is called a ligand.

Example: In the complex ion \({\left[ {{\rm{Fe(CN}}{{\rm{)}}_{\rm{6}}}} \right]^{{\rm{4 – }}}}\), the central \({\rm{F}}{{\rm{e}}^{{\rm{2 + }}}}\) ion is attached to the six \({\rm{C}}{{\rm{N}}^{\rm{ – }}}\) ligands.

Types of Isomerism in Coordination Compounds

The phenomenon in which two or more substances having the same molecular formula possess either different chemical structures or different spatial arrangements of atoms or groups is called isomerism, and such substances are called isomers.

Coordination compounds show two types of isomerism, i.e., structural isomerism and stereoisomerism.

1. Structural isomerism: It is due to the different ionisation behaviour, the difference in the position of ligand and the different modes of linkage of ligands. The different types of structural isomerism are:
   1. Ionisation isomerism
   2. Hydrate or solvate isomerism
   3. Linkage isomerism
   4. Coordinate isomerism
   5. Ligand isomerism
2. Stereoisomerism: It is due to the stereochemical disposition of ligands around the central metal atom or ion. The different types of stereoisomerism are:
   1. Geometrical isomerism
   2. Optical isomerism

Ionisation Isomerism

The coordinate compound having the same molecular formula but showing a different ionisation behaviour and furnishing different ions in the solution is called ionisation isomers. The phenomenon is called ionisation isomerism.

Example 1: \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{Br}}} \right]{\rm{S}}{{\rm{O}}_4}\) i.e. Pentaaminenitritocobalt \(\left( {{\rm{III}}} \right)\) sulphate and \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}\left( {{\rm{S}}{{\rm{O}}_4}} \right)} \right]{\rm{Br}}\) i.e., pentaaminesulphatocobalt (III) bromide. These ionise as follows,

The complex \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{Br}}} \right]{\rm{S}}{{\rm{O}}_4}\) is red-violet, and its aqueous solution gives a white precipitate of \({\rm{BaS}}{{\rm{O}}_4}\) with \({\rm{S}}{{\rm{O}}_4}^{2 – }\) thus it forms the presence of \({\rm{BaC}}{{\rm{l}}_2}\) ions in the solution. Whereas, \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}\left( {{\rm{S}}{{\rm{O}}_4}} \right)} \right]{\rm{Br}}\) is red, and its aqueous solution gives a light yellow precipitate \({\rm{AgBr}}\) of with  \({\rm{AgN}}{{\rm{O}}_3}\) , thus confirming the presence of \({\rm{B}}{{\rm{r}}^ – }\) ions in its solution.

Example 2: \(\left[ {{\rm{Pt}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{{({\rm{OH}})}_2}} \right]{\rm{S}}{{\rm{O}}_4}\) and \(\left[ {{\rm{Pt}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{S}}{{\rm{O}}_4}} \right]{({\rm{OH}})_2}\)

Example 3: \(\left[ {{\mathop{\rm Pt}\nolimits} {{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{C}}{{\rm{l}}_2}} \right]{\rm{B}}{{\rm{r}}_2}\) and \(\left[ {{\rm{Pt}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{B}}{{\rm{r}}_2}} \right]{\rm{C}}{{\rm{l}}_2}\)

Example 4: \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{N}}{{\rm{O}}_3}} \right]{\rm{S}}{{\rm{O}}_4}\) and \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}{\rm{S}}{{\rm{O}}_4}} \right]{\rm{N}}{{\rm{O}}_3}\)

Hydrate or Solvate Isomerism

Compounds with the same composition but differ in the number of solvent molecules present in the ligand and as a solvent molecule in a crystal lattice is called solvate isomers. The phenomenon is called hydrate isomerism. If water is the solvent, these are called hydrate isomers.

Example 1: Three hydrate isomer of hexahydrate of chromic chloride with empirical formula \({\rm{CrC}}{{\rm{l}}_3} \cdot 6{{\rm{H}}_2}{\rm{O}}\) are as follows,

1. \(\left[ {{\rm{Cr}}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_3}\) having three ionic chlorides is violet in colour.
2. \(\left[ {{\rm{Cr}}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_5}{\rm{Cl}}} \right]{\rm{C}}{{\rm{l}}_2} \cdot {\mkern 1mu} {{\rm{H}}_2}{\rm{O}}\) having two ionic chlorides is blue-green.
3. \(\left[ {{\rm{Cr}}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_4}{\rm{C}}{{\rm{l}}_2}} \right]{\rm{Cl}} \cdot 2{{\rm{H}}_2}{\rm{O}}\) having one ionic chloride is dark green.

Example 2: \(\left[ {{\rm{CoCl}}\left( {{{\rm{H}}_2}{\rm{O}}} \right){{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{C}}{{\rm{l}}_2}\) and \(\left[ {{\rm{CoC}}{{\rm{l}}_2}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{Cl}}.{{\rm{H}}_2}{\rm{O}}\)

Example 3: \(\left[ {{\rm{CoCl}}\left( {{{\rm{H}}_2}{\rm{O}}} \right){{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{B}}{{\rm{r}}_2}\) and \(\left[ {{\rm{CoB}}{{\rm{r}}_2}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{Cl}}.{{\rm{H}}_2}{\rm{O}}\)

Linkage Isomerism

Linkage isomerism is observed in the compounds containing ambidentate ligands, i.e., when more than one atom in a unidentate ligand may function as a donor.

Example 1: The pentaamminenitrocobalt \(\left( {{\rm{III}}} \right)\) ion, i.e., \({\left[ {{\rm{Co}}\left( {{\rm{N}}{{\rm{O}}_2}} \right){{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}} \right]^{2 + }}\)and pentaamminenitritocobalt \(\left( {{\rm{III}}} \right)\) ion, i.e., \({\left[ {{\rm{Co}}({\rm{ONO}}){{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}} \right]^{2 + }}\). In \({\rm{NO}}_2^ – \) either nitrogen or an oxygen atom may act as a donor giving to different isomers.

Example 2: \({\left[ {{\rm{Cr}}\left( {{\rm{SCN}}} \right){{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_5}} \right]^{2 + }}\) and \({\left[ {{\rm{Cr}}\left( {{\rm{NCS}}} \right){{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_5}} \right]^{2 + }}\)

Coordination Isomerism

The isomerism due to the interchange of ligands between the positive and negative part of the coordination sphere is called coordination isomerism. Coordination isomerism is shown by the complexes in which both positive and negative parts are complex species.

Example 1: \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]\left[ {{\rm{Cr}}{{\left( {{{\rm{C}}_2}{{\rm{O}}_4}} \right)}_3}} \right]\) and \(\left[ {{\rm{Co}}{{\left( {{{\rm{C}}_2}{{\rm{O}}_4}} \right)}_3}} \right]\left[ {{\rm{Cr}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]\)

Example 2: \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]\left[ {{\mathop{\rm Cr}\nolimits} {{({\rm{CN}})}_6}} \right]\) and \(\left[ {{\rm{Co}}{{({\rm{CN}})}_6}} \right]\left[ {{\rm{Cr}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]\)

Example 3: \(\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]\left[ {{\rm{PtC}}{{\rm{l}}_4}} \right]\) and \(\left[ {{\rm{Pt}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]\left[ {{\rm{CuC}}{{\rm{l}}_4}} \right]\)

Example 4: \(\left[ {{\rm{Co}}{{({\rm{en}})}_3}} \right]\left[ {{\rm{Cr}}{{({\rm{CN}})}_6}} \right]\) and \(\left[ {{\rm{Cr}}{{({\rm{en}})}_3}} \right]\left[ {{\rm{Co}}{{({\rm{CN}})}_6}} \right]\)

Ligand Isomerism

The isomerism due to the isomeric form of the ligand is called ligand isomerism.

Example: The ligand diaminopropane can exist as \({\rm{1, 2}}\)-diaminopropane represented as pn and \({\rm{1, 3}}\)-diaminopropane, represented as tn. The ligand isomers are \({\left[ {{\rm{Co}}{{\left( {{{\rm{p}}_{\rm{n}}}} \right)}_2}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\) and \({\left[ {{\rm{Co}}{{\left( {{{\rm{t}}_{\rm{n}}}} \right)}_2}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\)

Stereoisomerism

Different atoms or groups of atoms occupying different spatial positions around the central metal atom are called stereoisomers of coordination compounds. Stereoisomerism is due to the directional nature of coordinate bonds in coordination compounds. Geometrical isomerism and optical isomerism are two types of stereoisomerism.

Geometrical Isomerism in Coordination Compounds

Geometric isomerism is observed in heteroleptic complexes. In geometrical isomerism, the spatial arrangement of groups around the central metal atom is different. Similar groups are arranged on the same side of the central metal atom called cis isomers. The phenomenon is called cis isomerism. Similar groups arranged on the opposite side of the central metal atom are called trans-isomers. The phenomenon is called trans isomerism. Therefore, it is called cis-trans isomerism. Generally, the \({\rm{X}}\)-ray diffraction method is used to distinguish cis and trans isomers.

Geometric isomerism is not possible for the complexes with coordination number \({\rm{2}}\) and \({\rm{3}}\) and tetrahedral complexes with coordination number \({\rm{4}}\) because all the \({\rm{4}}\) positions are equivalent in this. However, geometric isomerism is quite common in the square planar and octahedral complexes.

Geometrical Isomerism in Square Planar Complexes

Square planar complexes have coordination number \({\rm{4}}\). This central metal is represented as \({\rm{M}}\) and the unidentate ligands as \({\rm{A,}}\,{\rm{B,}}\,{\rm{C,}}\,{\rm{D}}\). etc. The square planar complexes are classified into different types like \({\rm{M}}{{\rm{A}}_2}{{\rm{B}}_2},\,{\rm{M}}{{\rm{A}}_2}{\rm{BC}},\,{\rm{M}}{\left( {{\rm{AB}}} \right)_2}\) and \({\rm{MABCD}}\).

\({\rm{M}}{{\rm{A}}_2}{{\rm{B}}_2}\) type:

Example 1: Diamminedichloroplatinum \(\left( {{\rm{II}}} \right)\), i.e.,  \(\left[ {{\rm{Pt}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_2}{\rm{C}}{{\rm{l}}_2}} \right]\)

Example 2: \(\left[ {{\rm{Pd}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_2}{{\left( {{\rm{N}}{{\rm{O}}_2}} \right)}_2}} \right]\)

Example 3: Dichlorobis(pyridine)platinum(II) i.e., \(\left[ {{\rm{PtC}}{{\rm{l}}_2}{{\left( {{\rm{py}}} \right)}_2}} \right]\)

\({\rm{M}}{{\rm{A}}_{\rm{2}}}{\rm{BC}}\) Type

Example: \(\left[ {{\rm{Pt}}\left( {{\rm{N}}{{\rm{H}}_3}} \right){\rm{Cl}}{{\left( {{\rm{py}}} \right)}_2}} \right]\)

\({\rm{M(AB}}{{\rm{)}}_{\rm{2}}}\) Type

In which \({\rm{AB}}\) represents an unsymmetrical bidentate ligand.

Example: \(\left[ {{\rm{Pt}}{{({\rm{gly}})}_2}} \right]\) where \({\rm{gly}} = {\rm{N}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CO}}{{\rm{O}}^ – }\) is glycinato.

\({\rm{MABCD}}\) Type

It forms three isomers. Their structures may be obtained by fixing the position of one ligand like \({\rm{A}}\) and placing at the trans position any one of the remaining three ligands one by one.

Example: \(\left[ {{\rm{PtBrCl}}\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right){\rm{(py)}}} \right]\)

Geometrical Isomerism in Octahedral Complexes

Octahedral complexes have coordination number \(6\). A regular octahedral contains eight phases and six equivalent vertices. In an octahedral complex, metal is placed at the centre and six ligands at the vertices. Octahedral complexes of type \({\rm{M}}{{\rm{A}}_6},\,{\rm{M}}{{\rm{A}}_5}{\rm{B}},\) and \({\rm{MA}}{{\rm{B}}_5}\) do not show geometric isomerism because, in these complexes, different spatial arrangements of ligands are not possible.

Some common type of geometrical isomerism of octahedral complexes are \({\rm{M}}{{\rm{A}}_4}{{\rm{B}}_2}\) or \({\rm{M}}{{\rm{A}}_2}{{\rm{B}}_4},\,\left[ {{\rm{M}}{{\left( {{\rm{AA}}} \right)}_2}{{\rm{B}}_2}} \right]\) or \(\left[ {{\rm{M}}{{\left( {{\rm{AA}}} \right)}_2}{\rm{BC}}} \right]\), and \({\rm{M}}{{\rm{A}}_3}{{\rm{B}}_3}\)

\({\rm{M}}{{\rm{A}}_4}{{\rm{B}}_2}\) or \({\rm{M}}{{\rm{A}}_2}{{\rm{B}}_4}\) type

Examples 1: \({\left[ {{\rm{Cr}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\) ion

Examples 2: \({\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\) ion

\(\left[ {{\rm{M}}{{({\rm{AA}})}_2}{{\rm{B}}_2}} \right]\) or \(\left[ {{\rm{M(AA}}{{\rm{)}}_{\rm{2}}}{\rm{BC}}} \right]\) type: It contains one symmetrical bidentate ligand \({{\rm{(AA)}}}\), and the other two unidentate ligands may be the same or different.

Example: \({\left[ {{\rm{Co}}{{({\rm{en}})}_4}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\) ion

\({\rm{M}}{{\rm{A}}_3}{{\rm{B}}_3}\) type:

Example: \({\rm{RhC}}{{\rm{l}}_3}{({\rm{py}})_3}\)

In cis-form, similar groups occupy adjacent positions at the corners of one of the octahedral faces, called facial or fac-isomer. In trans-form, the positions of the trio of donor atoms are around the meridian of the octahedron. Hence, it is called meridonial or mer-isomer.

Optical Isomerism in Coordination Compounds

Optical isomerism is shown by the chiral compounds, i.e., compounds that do not possess any symmetry element. Due to chirality, mirror images are not superimposable on the compounds themselves. Hence the compounds are optically active. The two forms of the molecule, i.e., \({\rm{d}}\) (dextro) and \({\rm{l}}\) (laevo), mirror images of each other called enantiomers.

The d (Dextro) form rotates the plane polarised light in a clockwise direction (towards the right), and \({\rm{l}}\) (Laevo) form rotates the plane polarised light in an anticlockwise direction (towards left). The octahedral complexes showing optical isomerism are classified into \(\left[ {{\rm{M}}{{\left( {{\rm{AA}}} \right)}_3}} \right],\,\left[ {{\rm{M}}{{\left( {{\rm{AA}}} \right)}_2}{\rm{BC}}} \right]\) and \(\left[ {{\rm{M}}\left( {{\rm{AA}}} \right){{\rm{B}}_2}{{\rm{C}}_2}} \right]\) type.

\(\left[ {{\rm{M(AA}}{{\rm{)}}_{\rm{3}}}} \right]\) type: It contains three symmetrical bidentate ligands

Examples: \({\left[ {{\rm{Co(en}}{{\rm{)}}_{\rm{4}}}} \right]^{{\rm{3 + }}}}\) and \({\left[ {{\rm{Cr(ox}}{{\rm{)}}_{\rm{3}}}} \right]^{{\rm{3 – }}}}\)

\(\left[ {{\rm{M}}{{({\rm{AA}})}_2}{{\rm{B}}_2}} \right]\) or \(\left[ {{\rm{M}}{{\left( {{\rm{AA}}} \right)}_{\rm{2}}}{\rm{BC}}} \right]\) type: It contains two symmetrical bidentate ligands

Example: \({\left[ {{\rm{CoC}}{{\rm{l}}_2}{{({\rm{en}})}_2}} \right]^ + }\)

This trans-isomer does not show optical isomerism since it is symmetrical. In contrast, cis shows optical isomerism as it is unsymmetrical.

\(\left[ {{\rm{M}}({\rm{AA}}){{\rm{B}}_2}{{\rm{C}}_2}} \right]\) type: It contains one symmetrical bidentate ligands

Example:  \({\rm{Co}}({\rm{en}}){\left( {{\rm{N}}{{\rm{H}}_3}} \right)_2}{\rm{C}}{{\rm{l}}_2}\)

Summary

This article helped to understand different types of isomerism in a coordination compound. In this article, you acquired knowledge on the different types of structural isomerism, i.e., ionisation, hydrate, coordinate, linkage and ligand isomerism and stereoisomerism like geometrical and optical isomerism with suitable examples.

FAQs on Isomerism in Coordination Compounds

Q. What are the conditions for optical isomerism in coordination compounds?
Ans: The conditions for optical isomerism in coordination compounds are that compounds should be optically active, have a non-superimposable mirror image and do not exhibit a plane of symmetry.

Q. How do you find isomerism in a coordination compound?
Ans: Structural isomerism is identified based on different ionisation behaviour, the difference in ligand position and different modes of linkage of ligands. Stereochemical isomerism is studied based on the stereochemical distribution of ligands around the central metal atom or ion.

Q. What are the 2 types of isomers?
Ans: Isomers are mainly of two types, i.e., structural isomers and stereoisomers.

Q. What is isomerism in coordination compounds?
Ans: The phenomenon in which two or more substances having the same molecular formula possess either different chemical structures or different spatial arrangements of atoms or groups is called isomerism. In coordination compounds, it is due to the different ionisation behaviour, the difference in the position of ligands, different modes of linkage of ligands, etc.

Q. Which type of isomerism is shown by the two complexes?
a) [CoCl(H2O)(NH3)4]Cl2[CoCl(H2O)(NH3)4]Cl2 and [CoCl2(NH3)4]Cl.H2O
b) [Pt(NH3)Cl(py)2]
Ans: a) The complex [CoCl(H2O)(NH3)4]Cl2[CoCl(H2O)(NH3)4]Cl2 and [CoCl2(NH3)4]Cl.H2O[CoCl2(NH3)4]Cl.H2O shows hydrate isomerism.
b)The complex [Pt(NH3)Cl(py)2][Pt(NH3)Cl(py)2] shows geometrical isomerism.

Q. What is coordination isomerism? Explain with an example?
Ans: The isomerism due to the interchange of ligands between the positive and negative part of the coordination sphere is called coordination isomerism. Coordination isomerism is shown by the complexes in which both positive and negative parts are complex species.
Example 1: [Co(NH3)6][Cr(C2O4)3][Co(NH3)6][Cr(C2O4)3] and [Co(C2O4)3][Cr(NH3)6]

Q. Why is isomerism studied in coordination compounds?
Ans: Coordination compounds exist in different structural and stereoisomers with different properties. Therefore, isomerism is studied in coordination compounds.

Q. What are the different types of Structural Isomerism?
Ans: The different types of Structural Isomerism are Ionization Isomerism, Coordination Isomerism, Linkage Isomerism and Hydrate Isomerism.

Q. What are fac and mer isomers?
Ans: When three identical ligands occupy the vertices of a octahedron’s face, the isomers thus formed are called fac(ial) isomers. When the three ligands formed together with the central atom form a plane in the octahedron, the isomers are called mer(idional) isomers.

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