Question

Why do the d-block elements form coloured compounds?

Answer 1

1. Many d-block elements or transition metals, exhibit colour in their compounds or ions. This appearance is due to the presence of one or more unpaired electrons in the (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. Therefore, transition metal ions absorb radiation in the visible region and appear coloured.
4. The colour of ions of d-block elements depends on the number of unpaired electrons in the (n  − 1)d orbital. The ions having an equal number of unpaired electrons have similar colour.

Answer 2

The reason why d-block elements (transition elements) produce coloured compounds or have coloured ions is:

1. Presence of unpaired d-electrons.
2. d-d transition.
3. Nature of ligands attached to the metal ion.
4. Geometry of the complex formed by the metal ion. The colouration of metal ions such as \[\ce{MnO^-4}\], \[\ce{Cr2O^{2-}7}\], and \[\ce{CrO^-4}\] is attributed to charge transfer transitions, wherein an electron is temporarily transferred from oxygen to the metal ion.