Why do the transition metal ions exhibit characteristic colours in aqueous solution. Explain giving example.

Hydrated ions and compounds of many transition metals are coloured.
Ex. MnCl₂ in pink, CuCl₂ is blue, NiSO₄ is green, CuSO₄.5H₂O is blue coloured, etc.,
The colour of the hydrated transition metal ions and compounds may be attributed to the
presence of unpaired d-electrons.
In the gaseous state or isolated state of metal ion all the five d-orbitals in a subshell are degenerate.

But under the influence of approaching ligands (H₂O) five degenerate orbitals are splitted into
two sets of d-orbitals having different energies. This phenomenon is called crystal field splitting.
When white light falls on a hydrated transition metal ion having at least one unpaired d-electron, it absorbs a characteristic colour from visible region for d-d transition and transmit the complementary colour.
For example, [Ti(H₂O)₆]³⁺ has one 3d electron. It absorb green and yellow light ($\mathrm{\lambda }$ =500–570nm) from visible region and transmits a mixture of red and violet light ($\mathrm{\lambda }$ = 400–450 nm). Hence hydrated Ti³⁺ ion has purple colour. Similarly hydrated cupric ion (Cu²⁺) abosorbs orange red light from visible region and transmits its complementary colour greenish blue. Thus, cupric compounds have greenish blue colour.