Ferrocene will be well known to made members of this site. It consists of two cyclopentadienyl anions sandwiching an iron-II ion.
During my first year organic chemistry course (about 40 years ago) I remember a friend suggesting that the fluorene anion had a similar stability and it might be possible to make ferrocene type compounds with an iron-II ion sandwiched between two fluorene anions.
A group of us persuaded a sympathetic professor to let us have a go, and as I recall we prepared the anion by adding lithium aluminium hydride to solution of fluorene in ether cooled with dry ice then adding iron-II chloride. The iron chloride immediately went into solution, so some form of complex was formed, but we couldn't get much further as on warming to room temperature we got a hideous mess of many different products and we didn't have time to analyse it thoroughly.
So my question is whether these types of fluorene compound are known, or alternatively whether it is known that they cannot be prepared? I have attempted to Google this, but there is such a vast literature on ferrocene type compounds that I have not been able to find anything relevant.