Question

List various types of isomerism possible for coordination compounds, giving an example of each.

Answer 1

(i) Geometric isomerism: This type of isomerism is common in heteroleptic complexes. It arises due to the different possible geometric arrangements of the ligands. For example:

\[\begin{array}{cc}
\phantom{..}\ce{Cl}\phantom{.....}\ce{NH3}\phantom{}\\
\backslash\phantom{...}/\\
\ce{Pt}\\
/\phantom{...}\backslash\\
\ce{\underset{cis}{\phantom{..}Cl\phantom{.....}NH3}}
\end{array}\]

 

\[\begin{array}{cc}
\phantom{..}\ce{Cl}\phantom{.....}\ce{NH3}\phantom{}\\
\phantom{}\backslash\phantom{...}/\\
\ce{Pt}\\
/\phantom{...}\backslash\phantom{}\\
\ce{\underset{trans}{NH3\phantom{....}Cl\phantom{..}}}
\end{array}\]

(ii) Optical isomerism: This type of isomerism arises in chiral molecules. Isomers are mirror images of each other and are non-superimposable.

(iii) Linkage isomerism: This type of isomerism is found in complexes that contain ambidentate ligands. For example, [Co(NH₃)₅(NO₂)]Cl₂, which is obtained as a red form and [Co(NH₃)₅(ONO)]Cl₂ obtained as a yellow form.

(iv) Coordination isomerism: This type of isomerism arises when the ligands are interchanged between cationic and anionic entities of different metal ions present in the complex. For example, [Co(NH₃)₆] [Cr(CN)₆] and [Cr(NH₃)₆] [Co(CN)₆].

(v) Ionization isomerism: This type of isomerism arises when a counter ion replaces a ligand within the coordination sphere. Thus, complexes that have the same composition but furnish different ions when dissolved in water are called ionization isomers. For example, [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.

(vi) Solvate isomerism: Solvate isomers differ by whether or not the solvent molecule is directly bonded to the metal ion or merely present as a free solvent molecule in the crystal lattice. For example, [Cr(H₂O)₆]Cl₃ (violet) and [Cr(H₂O)₅Cl]Cl₂.H₂O (grey-green).