= Solution
In ionic counting, two $\eta^5$ cyclopentadienyl <anions> donate $12$ <electrons>. Therefore \b[<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 \b[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.
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