Why do transition metals form coloured complexes ?

Explanation

(1) The colour is due to the presence of one or more unpaired electrons in (n - 1) d-orbital. The transition metals have incompletely filled (n-1) d-orbitals.
(2) The energy required to promote one or more electrons within the d-orbitals involving d-d transitions is very low.
(3) The energy changes for d-d transitions lie in visible region of
electromagnetic radiation.
(4) Therefore transition metal ions absorb the radiation in the visible region and appear coloured.
(5) Colour of ions of d-block elements depends on the number of unpaired electrons in (n – 1) d-orbital. The ions having equal number of unpaired electrons have similar colour.
(6) The colour of metal ions is complementary to the colour of the radiation absorbed.