Question

(i) Give a chemical test to show that \([Co(NH_3)_5SO_4]Cl\) and \([Co(NH_3)_5Cl]SO_4\) are ionisation isomers.

(ii) What is meant by the 'Chelate effect'? Give an example.

Hint:

Ionisation isomers are coordination compounds that have the same formula but give different ions when dissolved in water, because the groups inside the complex (coordination sphere) and the groups outside (counter ions)

Answer 1

Concept:
Ionisation isomers are coordination compounds that have the same formula but give different ions when dissolved in water, because the groups inside the complex (coordination sphere) and the groups outside (counter ions) are swapped. We can tell them apart by testing for the free ion that each one releases in solution.

Step 1 (part i - the test):
In \([Co(NH_3)_5SO_4]Cl\), the sulphate is inside the complex and chloride is the free ion. In \([Co(NH_3)_5Cl]SO_4\), the chloride is inside the complex and sulphate is the free ion. So we test for the free ions.
Add aqueous barium chloride (\(BaCl_2\)): \([Co(NH_3)_5Cl]SO_4\) releases free \(SO_4^{2-}\) ions and gives a white precipitate of \(BaSO_4\). \([Co(NH_3)_5SO_4]Cl\) has no free sulphate, so it gives no precipitate with \(BaCl_2\).
Alternatively, add aqueous silver nitrate (\(AgNO_3\)): \([Co(NH_3)_5SO_4]Cl\) releases free \(Cl^-\) and gives a white precipitate of AgCl, while the other isomer does not.

Step 2 (part ii - chelate effect):
The chelate effect is the extra stability that a complex gains when a metal ion is bonded to a ligand that grips it at two or more points, forming a ring. Such ring-forming ligands (called chelating ligands) make the complex much more stable than a similar complex made from single-point (monodentate) ligands. The main reason is the increase in disorder (entropy): when one chelating ligand bonds, it pushes out several smaller ligands, increasing the number of free particles in solution.
Example: ethylenediamine (en) bonds to a metal through its two nitrogen atoms, so a complex like \([Co(en)_3]^{3+}\) is more stable than \([Co(NH_3)_6]^{3+}\).

Answer:
(i) Add \(BaCl_2\) solution: \([Co(NH_3)_5Cl]SO_4\) gives a white precipitate of \(BaSO_4\) (free sulphate), while \([Co(NH_3)_5SO_4]Cl\) does not. (Or add \(AgNO_3\): the latter gives a white AgCl precipitate from its free chloride.)
(ii) The chelate effect is the extra stability of a complex formed when a di- or polydentate ligand forms a ring with the metal ion, for example en in \([Co(en)_3]^{3+}\).