# How can different no. of unpaired electrons lead to different colours of co-ordination compounds?

To make the question clear I would like to take an example. $\ce{K_3[Fe(CN)6]}$ and $\ce{K3[FeF6]}$ shows different colors in solutions. It is given in my textbook that it is because of different number of unpaired electrons. But, how can the no. of unpaired electrons affect the frequency of light emitted?

The colors of co-ordination compounds are due to d-d transition of electrons. $\ce{CN^-}$ is a strong field ligand while $\ce{F^-}$ is a weak field ligand. Thus, there will be difference in the crystal field splitting energy in both of the given compounds. This, results in a different energy gap between the eg and t2g energy levels in them. Thus, when electrons undergo transition different frequencies corresponding to different energy gap is emitted.

I was hoping this as the answer. But, in my textbook the reason is attributed to only the difference in no. of unpaired electrons. Why is it so?

• The crystal field splitting does definitely contribute. But it is an unpaired electron which usually shifts between various available orbitals and paired electrons rarely do so. Moreover, there might also be spin inversions (see Hyperfine structure) which might give additional lines in the spectra. Presence of unpaired electrons helps in producing additional energy levels in electric and magnetic fields (zeeman and stark effect) providing additional lines in the spectra. All these provide establish the uniqueness of the spectral distribution radiated, and hence the colour. – Satwik Pasani Jan 11 '14 at 10:39
• @SatwikPasani Thankyou.....now I understood the concept behind it.Can you lease post this as an answer with any further explanations(if you would like to add) so that I can accept it as the answer. – Rajath Krishna R Jan 11 '14 at 12:42