= Solution
One equivalent of butyllithium removes one proton per hydrocarbon, giving the \b[hydropentalenyl monoanion $\mathrm{C_8H_7^-}$]. It does not give the doubly deprotonated <pentalenide>. <salt metathesis> with half an equivalent of iron(II) chloride therefore gives the sandwich complex
$$
\boxed{\mathrm{Fe}(\eta^5\!\text{-}\mathrm{C_8H_7})_2,\qquad d^6+6+6=18.}
$$
Each <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 \b[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.<Bis(η⁵-hydropentalenyl)iron by Elies Molins and others>[Acta Crystallographica C 52, 2414–2416: parallel coordinated rings and double-bond isomer disorder]{ref}
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