Question

Giving suitable examples, discuss the various types of structural isomerism exhibited by coordination compounds.

Answer 1

In structural isomerism, two or more complex molecules with the same chemical formula have distinct structural configurations. Coordination compounds’ structural isomerism can be classified into the following classes.

1. Ionisation isomerism: It occurs when two or more compounds have the same molecular formula but give different ions in solution. This happens when a ligand can either be part of the coordination sphere or an external counterion.
   For example, [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅SO₄]Cl:
   • Both compounds have the same molecular formula [Co(NH₃)₅Cl]SO₄, but the counterions Cl⁻ and SO₄²⁻ are swapped.
   • When dissolved in solution, they will produce different ions. The first will produce Cl⁻ and SO₄²⁻, while the second will produce Cl⁻ and SO₄²⁻.
     
2. Hydrate isomerism: Water molecules can act as both ligands and plain water molecules during crystallisation. When two complexes differ in this way, they show hydrate isomerism.
   For example, using the formula. CrC1₃·6H₂O, following three isomers are known to exist.
   • [Cr(H₂O)₆] Cl₃ hexaaquachromium (III) chloride (violet)
   • [Cr(H₂O)₅Cl]Cl₂·H₂O pentaaquachloridochromium (III) chloride monohydrate (blue-green)
   • [Cr(H₂O)₄Cl₂] Cl·2H₂O tetraaquadichloridochromium (III) chloride dihydrate (dark green)
     
3. Linkage or structural salt isomerism: They arise when a ligand can bind to the central metal ion through two different atoms. This occurs with ambidentate ligands, which have two donor atoms capable of coordinating with the metal ion.
   For example, in a NO₂ ion, both the N and O atoms can coordinate with the centre metal atom. Thus, the following pair of chemicals shows linkage isomerism.
   
4. Coordination isomerism: This type of isomerism can be seen in complexes where both positive and negative sections are complex species. The isomerism results from the exchange of ligands between the coordination spheres of positive and negative portions. Some couples with this form of isomerism are listed below.
   [Co(NH₃)₆] [Cr(C₂O₄)₃] and [Co(C₂O₄)₃] [Cr(NH₃)₆] 
   [Co(NH₃)₆] [Cr(CN)₆] and [Co(CN)₆] [Cr(NH₃)₆]
   [Cu(NH₃)₄] [PtC1₄] and [Pt(NH₃)₄] [CuC1₄]
   The ligands in these situations have been changed within the coordination spheres.
   For example, in the last situation, the initial complex contains the cation [Cu(NH₃)₄]²⁺ and the anion [PtC1₄]²⁻. Its coordination isomer contains the cation [Pt(NH₃)₄]²⁺ and the anion [CuC1₄]²⁻. Thus, the ligands coupled to Cu and Pt have been modified.
5. Ligand isomerism: This type of isomerism occurs when the ligand in the complex can exist in several isomeric forms.
   For example, the ligand diaminopropans can exist as both 1,2-diaminopropane and 1,3-diaminopropane.
   1. \[\begin{array}{cc}
      \ce{CH2 - CH - CH3}\\
      |\phantom{.......}|\phantom{.........}\\
      \phantom{..}\ce{NH2}\phantom{....}\ce{NH2}\phantom{........}\\
      \ce{1,2-diaminopropane (p_n)}
      \end{array}\]
   2. \[\begin{array}{cc}
      \ce{CH2 - CH2 - CH2}\\
      \phantom{.}|\phantom{............}|\phantom{...}\\
      \phantom{..}\ce{NH2}\phantom{.........}\ce{NH2}\phantom{..}\\
      \ce{1,3-diaminopropane (t_n)}
      \end{array}\]