Question

Of the complex ions, $\left[Co\right(H_{2}O{)}_6{]}^{3+}$ and $\left[Co\right(en{)}_3{]}^{3+}$, one has a yellow colour in aqueous solution and the other blue. Match each ion with a expected colour and state your reason for doing so.

Answer 1

$\left[Co\right(H_{2}O{)}_6{]}^{3+}$ is blue in color and $\left[Co\right(en{)}_3{]}^{3+}$ is yellow in color.

The color of coordination complexes arises from electronic transitions between levels whose spacing corresponds to the wavelengths available in the visible light. In complexes, these transitions are frequently referred to as d-d transitions because they involve the orbitals that are mainly d in character (for examples: $t_{2g}$ and $e_g$ for the octahedral complexes and $e_g$ and $t_{2g}$ for the tetrahedral complexes).

The colors exhibited are intimately related to the magnitude of the spacing between these levels. This spacing depends on factors such as the geometry of the complex, the nature of the ligands, and the oxidation state of the central metal atom.