Why do Cp2Mg and NaCp complexes turn pink when oxidized?

Question

I am just wondering why the complexes sodium cyclopentadienide ($\ce{NaCp}$) and cyclopentadienyl magnesium ($\ce{Cp2Mg}$) turn pink when oxidized? Where does the pink color come from?

Answer 1

With highly electroposive metals we may treat the cyclopentadienyl as being essentially an anion, $\ce{C5H5^-}$. When this is oxidized to form the radical, an extra electronic vacancy is opened up in one of the bonding orbitals, and another bonding orbital that is slightly lower in energy can transfer an electron into it with visible light absorption.

Let's play around with the Hückel model to get some estimates. For the anion, where all bonding orbitals are filled, the only possible transition in the ion is from a bonding orbital to an antibonding one, and the minimum transition energy is calculated as $2.24|\beta|$. From the observed stabilization of benzene we may put in $|\beta|\approx 1.5$ eV, rendering the transition energy estimate above 3 eV and into the ultraviolet range. We are not likely to see color on this account.

Now let's consider the radical formed by taking an electron away, with a vacancy in the higher of the two bonding levels. An electron can transition into this position from the lower bonding orbital with an estimated energy of only $1.38|\beta|$, which is now between 2.1 and 2.2 eV. This is in the short wavelengths of visible light, so we could see a yellowish to reddish color.