How does cyclopentadienyl anion act as reducing agent?

I am a second year university student, so I'm afraid my chemistry isn't great. I am looking for an explanation of how the $\ce{Cp-}$ anion acts as a reducing agent.


To contextualise - in a simple synthesis of ferrocene, a hydrated salt of $\ce{FeCl3}$ (the hexahydrate) was mistakenly added to a dry solution of LiCp in THF, which had been prepared by reacting n-BuLi and cyclopentadiene under nitrogen. Clearly anhydrous $\ce{FeCl2}$ was what was supposed to be added.


So far I know that a re-protonation of $\ce{Cp-}$ would occur upon addition of a hydrate to regenerate CpH and thus ruin the reaction.


However, the reaction was allowed to continue and some ferrocene was collected by vacuum sublimation, and therefore I know that something present must have reduced the $\ce{Fe^3+}$ to $\ce{Fe^2+}$ in order for some ferrocene to form. I assume this was the $\ce{Cp-}$ anion as I know this is oxidised by the air. I would expect this to be oxidation under basic conditions because of the LiOH created in the re-protonation step.


I am looking for the half-equation for the oxidation of $\ce{Cp-}.$
What does cyclopentadienyl anion oxidise to? Why are the cyclopentadienyl salts air-sensitive?




**Disclaimer**: None of this constitutes proof; it's just an informed guess. Any corrections with reputable sources are welcome.




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The simplest proposal seems to be single-electron transfer (SET) from $\ce{Cp-}$ to $\ce{Fe^3+}$. This forms the neutral cyclopentadienyl *radical* as well as $\ce{Fe^2+}$. This $\ce{Fe^2+}$ then reacts with any remaining unreduced $\ce{Cp-}$ to form ferrocene $\ce{FeCp2}$.


All in all for a balanced equation, we would need three equivalents of $\ce{Cp-}$ per equivalent of $\ce{Fe^3+}$. One equivalent for the redox reaction / SET:


$$\ce{Fe^3+ + Cp- -> Fe^2+ + Cp^.}$$


and two equivalents to form ferrocene:


$$\ce{Fe^2+ + 2Cp- ->FeCp2}$$


The radicals probably go off and dimerise, which leads to dihydrofulvalene $\ce{Cp-Cp}$. (Basically, [fulvalene](https://en.wikipedia.org/wiki/Fulvalene), but with the middle double bond changed to a single bond.)


There is some support for this hypothesis, namely the accidental synthesis of ferrocene from $\ce{FeCl3}$ and $\ce{CpMgBr}$ (see [Wikipedia](https://en.wikipedia.org/wiki/Ferrocene#Via_Grignard_reagent)), which Buttonwood also mentioned in the comments. This follows almost the same scheme as outlined above. In the present case we have $\ce{CpLi}$ instead of $\ce{CpMgBr}$, and there are definitely differences between Grignards and organolithiums, but to a first approximation I'd expect similar behaviour.


Organolithiums are known to sometimes react as single-electron reducing agents, too (e.g. [lithium–halogen exchange sometimes involves SET](https://pubs.acs.org/doi/10.1021/jo00383a023)) so it's hardly implausible that $\ce{CpLi}$ might behave similarly.


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