(i) CHEMICAL TEST FOR IONISATION ISOMERS

Step 1 - Understand ionisation isomerism.
Ionisation isomers have the same molecular formula but give DIFFERENT IONS in solution,
because a group that is a ligand in one isomer is the counter ion in the other.

Step 2 - Write the ionisation of each compound in water.
$$[Co(NH_3)_5SO_4]Cl \longrightarrow [Co(NH_3)_5SO_4]^+ + Cl^-$$
(gives FREE chloride ion; the sulphate is locked inside the coordination sphere)
$$[Co(NH_3)_5Cl]SO_4 \longrightarrow [Co(NH_3)_5Cl]^{2+} + SO_4^{2-}$$
(gives FREE sulphate ion; the chloride is locked inside the coordination sphere)

Step 3 - Test with silver nitrate.
Add $AgNO_3$ solution to each.
$[Co(NH_3)_5SO_4]Cl$ : gives a WHITE PRECIPITATE of $AgCl$ (soluble in $NH_4OH$)
$$Cl^- + Ag^+ \rightarrow AgCl \downarrow \ (\text{white})$$
$[Co(NH_3)_5Cl]SO_4$ : gives NO precipitate with $AgNO_3$.

Step 4 - Test with barium chloride.
Add $BaCl_2$ solution to each.
$[Co(NH_3)_5Cl]SO_4$ : gives a WHITE PRECIPITATE of $BaSO_4$ (insoluble in dil. HCl)
$$SO_4^{2-} + Ba^{2+} \rightarrow BaSO_4 \downarrow \ (\text{white})$$
$[Co(NH_3)_5SO_4]Cl$ : gives NO precipitate with $BaCl_2$.

(ii) CHELATE EFFECT

Definition: When a DI- or POLY-DENTATE ligand binds to the same central metal ion
through two or more of its donor atoms, it forms a closed RING (usually 5- or
6-membered) called a CHELATE. Such a chelated complex is MUCH MORE STABLE than a
comparable complex containing the same number of monodentate ligands. This extra
stabilisation is called the CHELATE EFFECT.

Reason: chelation is favoured mainly by a large POSITIVE ENTROPY change, because one
chelating ligand displaces several monodentate ligands, increasing the number of free
particles in solution.

Example: $[Ni(en)_3]^{2+}$ (en = ethane-1,2-diamine, a bidentate ligand forming three
five-membered rings) is far more stable than $[Ni(NH_3)_6]^{2+}$, even though the donor
atom is nitrogen in both cases.
Another example: EDTA$^{4-}$, a hexadentate ligand, forms extremely stable chelates such
as $[Ca(EDTA)]^{2-}$, which is used to estimate the hardness of water and to treat
lead poisoning.