Explain why compounds of Fe^2+ are coloured in solution. (4 marks)

Fe²⁺ has an electron configuration of 1s²2s²2p⁶3s²3p⁶3d⁶. When the ligands of the solution bind to the metal ion the d-orbitals are split which creates an energy gap that allows for electron transitions. The d-electrons absorb light of energy equal to the energy gap. Electrons in lower energy orbitals are promoted to higher energy orbitals. The wavelengths of light that are not absorbed by the electrons are transmitted which give the solution a green colour.