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A ferrocene analogue with a pyrrolyl ring replacing one cyclopentadienyl ring, $\mathrm{Fe}({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5)({\eta }^5-{\mathrm{C}}_4{\mathrm{H}}_4\mathrm{N})$. Both anionic rings are six-electron donors.

The sandwich compound $\mathrm{Ni}({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5{)}_2$, conventionally Ni(II), $d^8$, and 20-electron. The extra electrons relative to ferrocene occupy metal–ring antibonding orbitals.

In a metallocene, several orbitals with substantial metal $d$ character are nonbonding molecular orbitals or only weakly bonding or antibonding with respect to the strong Cp–metal framework. Adding or removing a small number of electrons from them can therefore change oxidation state or spin without destroying the principal ring–metal bonds. This allows stable compounds on either side of the 18-electron count: for example, the usual neutral counts for chromocene, ferrocene, cobaltocene and nickelocene are respectively 16, 18, 19 and 20. The 18-electron rule remains a useful reference, but occupation of a weakly antibonding orbital is much less disruptive than occupation of a strongly antibonding orbital. Redox stability also depends on geometry, substituents and reaction conditions, so this is a bonding explanation of the observed flexibility rather than a guarantee of equal stability for all counts.

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]

In ionic counting, two ${\eta }^5$ cyclopentadienyl anions donate $12$ electrons. Therefore ferrocene, cobaltocene and nickelocene contain 18, 19 and 20 valence electrons, respectively: Fe(II) is $d^6$, Co(II) $d^7$ and Ni(II) $d^8$. ferrocene fills the bonding and approximately nonbonding levels while leaving the next metal–ring antibonding pair empty. cobaltocene and nickelocene add one and two electrons to that antibonding set.

Increasing antibonding occupation lowers the net metal–ring bond order. It therefore explains the simultaneous increase in M–C distance and decrease in dissociation enthalpy from Fe to Co to Ni. The trend does not follow the shrinking size of the isolated metal ions; the orbital occupation is decisive here. The specified enthalpy separates the entire sandwich into a gaseous metal dication and two gaseous ring anions. It measures that complete ionic dissociation process, not ten independent localized M–C bond energies, so dividing it by ten is not a justified bond-energy determination.

Oxidizing cobaltocene removes its antibonding nineteenth electron and gives the particularly favourable 18-electron cobaltocenium cation. iodine can accept the electrons, giving cobaltocenium iodide at the simple stoichiometryEach cobalt changes from Co(II), $d^7$, to Co(III), $d^6$; each iodine atom changes from 0 to $-1$. Excess iodine can change the isolated counterion to a polyiodide such as ${\mathrm{I}}_3^{-}$. In contrast, oxidation of 18-electron ferrocene would remove an electron from its occupied closed-shell manifold to form 17-electron ferrocenium. iodine is insufficient to drive that reaction under the conditions implicit here. ferrocene can still be oxidized by suitable stronger oxidants; an electron count is a useful rationale for the comparative redox behaviour, not a universal proof of a numerical electrode potential.