Coordination compounds of transition metals are often COLOURED. CFT explains this colour as arising from

1. syllabus explains the origin of colour in transition-metal complexes. 2. In an octahedral field, the d-orbitals split into $t_{2g}$ (lower) and $e_g$ (higher) by an energy $\Delta_o$. 3. If $\Delta_o$ falls in the visible range ($\sim 1.7\text{–}3.1\,\text{eV}$, i.e. wavelengths $400\text{–}730\,\text{nm}$), the complex can ABSORB a visible photon that excites an electron from $t_{2g}$ to $e_g$ — a $d{-}d$ TRANSITION. 4. The remaining (unabsorbed) wavelengths show as the complex's colour. The colour observed is the COMPLEMENTARY colour of what is absorbed. 5. Example: $[\mathrm{Cu(H_2O)_6}]^{2+}$ absorbs orange light, so we see its complementary blue colour. 6. Options A, B, D have no physical basis for the systematic colour of transition-metal complexes. _Source: syllabus Chemistry Part 1, Ch 5, §5.6.3 (Colour in coordination compounds — d-d transitions), p. 13–14._