Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 2 / B8 / d / i / Solution

One equivalent of butyllithium removes one proton per hydrocarbon, giving the hydropentalenyl monoanion ${\mathrm{C}}_8{\mathrm{H}}_7^{-}$. It does not give the doubly deprotonated pentalenide. salt metathesis with half an equivalent of iron(II) chloride therefore gives the sandwich complexEach coordination ligand binds through its aromatic five-membered portion and retains an uncoordinated double bond in the fused ring. The orientation of this unsymmetrical fused appendage makes the two coordinated coordination ligand faces distinguishable. Pairing matching and opposite planar configurations gives a racemic form, $(R_p,R_p)/(S_p,S_p)$, and a meso form, $(R_p,S_p)$. This describes the two stereoisomeric classes, rather than assigning the two easily interconverting rotations of ordinary ferrocene as separate configurational isomers. The molecular structure and double-bond disorder of bis(hydropentalenyl)iron have been examined crystallographically.^[ref]