How do Covalent Bonds Produce Color?

[Johnleprekan]

I've learned about the Band Theory of metallic bonds and how they produce color, but how does a covalent bond produce color? Normally a metal such as iron absorbs and emits back the full range of colors due to metallic bonds, but when oxygen or sulfur get added it produces a different color. Can someone explain what happens here? One explanation for iron oxide and another for iron sulfate or iron sulfide, please. If possible explain in Layman's terms.

 

[DrDu]

The weak green colour of iron II sulfate is due to transitions within the d-orbitals on iron, ie, covalent bonding is not involved. The colour of the other substances is mainly due to so called charge transfer transitions. The bonding in these compounds is mainly ionic and a transfer of charge from the negatively charged ligand (e.g. sulfide in iron sulfide or oxide in iron oxide) to the metal requires both an energy corresponding to visible light and goes in line with a high transition dipole moment.

 

[Johnleprekan]

Thank you.