Question

Generally transition elements and their salts are colored due to the presence of unpaired electrons in metal ions. Which of the following compounds are colored?

• A. $KMn{O}_4$
• B. $Ce(S{O}_4{)}_2$
• C. $TiC{l}_4$
• D. $C{u}_{2}C{l}_2$

Answer 1

Answer: (A), (B)

$Ce(S{O}_4{)}_2$

It is common knowledge that $KMn{O}_4$ is coloured (violet-purple). This is despite the fact that the central metal atom doesn't have any $d$ electrons! The colour arises from a charge transfer reaction within the molecule, in which photons promote an electron from the highest energy molecular orbital in one of the $Mn-O$ bonds to an empty $d-orbital$ on $Mn$. This promotion corresponds to the energy of a yellow photon, which means that we observe the complimentary - purple!

similarly, $Ce(S{O}_4{)}_2$ is also coloured due to charge-transfer spectra. $TiC{l}_4$ and $C{u}_{2}C{l}_2$ are not coloured.