What is meant by 'disproportionation'? Give an example of disproportionation reaction in aqueous solution.
OR
Suggest reasons for the following features of transition metal chemistry:
(i) The transition metals and their compounds are usually paragmagnetic.
(ii) The transition metals exhibit variable oxidation states.

Explanation

Disproportionation reaction involves the oxidation and reduction of the same substance. The two examples of disproportionation reaction are :
(i) Aqueous `NH_3` when treated with `Hg_2 Cl_2` (solid) forms mercury aminochloride disproportionatively.
`HgCl+2NH_3toHg+Hg (NH_2)Cl+NH_4Cl`
(ii) `2Cu^(+) to Cu+Cu^(2+)`
OR
(i) Because transition metal atoms or ions have unpaired d-electrons in their congfiguration
(ii) Transition metals exhibit variable oxidation state because both ns and `(n-1)` d-electrons, both can take part in bond formation because the energy of `(n-1)` and `nx` orbitals are very close.