Question

Explain the coordination isomerism and ionisation isomerism with examples

Answer 1

Coordination compounds:

• A coordination complex may be defined as a compound that results from the combination of two or more stable chemical species and retains its identity in the solid as well as dissolved state.
• The cation to which one or more neutral molecules or ions are coordinated is called the central ion.
• The molecules or ions so attached are called ligands.
• The atom in the ligand which can donate the electron pair is called donar atom.
• The total number of ligands attached to a central ion is called the coordination number of that ion.
• The central ion together with molecules or ions coordinated to it constitute what is termed as coordination complex.

Isomerism in coordination compounds:

• The compounds having same molecular formula but different structure and hence different physical and chemical properties are called isomers.
• The phenomenon of the existence of such compounds is known as isomerism.

Coordination isomerism

• The coordination isomerism isomerism is observed in the case of compounds comprising of both cationic and anionic complexes
• In this isomerism, there is an exchange of ligands between cations and anions of different metal ions present in coordination compounds.
• The examples for coordination isomersm are Hexacyanochromium (III) hexaammine cobalt (III) $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]$ and Hexacyanocobalt (III) hexaammine chromium (III) $\left[\mathrm{Co}{\left(\mathrm{CN}\right)}_6\right]\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_6\right]$
• Here the Ammonia $\left({\mathrm{NH}}_3\right)$ and Cyanide $\left({\mathrm{CN}}^{-}\right)$ ligands exchanges their positions and the isomers are formed.

Ionization isomerism

• Ionisation isomerism involves the exchange of ions inside and outside the coordination sphere.
• These isomers give different ions in solution, hence it is called ionisation isomerism.
• The examples for ionization isomerism are Pentaamminesulfatocobalt(III)bromide $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{SO}}_4\right]\mathrm{Br}$ and Pentaamminebromidocobalt(III)sulfate $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Br}\right]{\mathrm{SO}}_4$
• Pentaamminebromidocobalt(III)sulfate $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Br}\right]{\mathrm{SO}}_4$ gives red-violet precipitate with ${\mathrm{BaCl}}_2$ indicating the sulphate ion is outside the coordination sphere.

$\underset{\mathrm{Pentaamminebromidocobalt}\left(\mathrm{III}\right)\mathrm{sulfate}}{\left[\mathrm{CoBr}{\left({\mathrm{NH}}_3\right)}_5\right]{\mathrm{SO}}_4}\to \underset{\mathrm{Pentaamminebromidocobalt}\left(\mathrm{III}\right)\mathrm{ion}}{{\left[\mathrm{CoBr}{\left({\mathrm{NH}}_3\right)}_5\right]}^{2+}}+\underset{\mathrm{Sulfate}\mathrm{ion}}{{\mathrm{SO}}_4^{2-}}$

• Pentaamminesulfatocobalt(III)bromide $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{SO}}_4\right]\mathrm{Br}$ gives red precipitate with ${\mathrm{BaCl}}_2$ but gives a precipitate of $\mathrm{AgBr}$ with Silver nitrate indicating that now bromide ion is outside the coordination sphere.

$\underset{\mathrm{Pentaamminesulfatocobalt}\left(\mathrm{III}\right)\mathrm{bromide}}{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{SO}}_4\right]\mathrm{Br}}\to \underset{\mathrm{Pentaamminesulfatocobalt}\left(\mathrm{III}\right)\mathrm{ion}}{{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{SO}}_4\right]}^{+}}+\underset{\mathrm{Bromide}\mathrm{ion}}{{\mathrm{Br}}^{-}}$

Therefore, coordination isomerism involves the exchange of ligands between cations and anions of different metal ions present in coordination compounds and Ionisation isomerism involves the exchange of ions inside and outside the coordination sphere.