Metallocene

A complex in which a metal is bound between two cyclopentadienyl-type rings, commonly with hapticity ${\eta }^5$ at each ring. In ionic electron counting, each unsubstituted cyclopentadienyl anion donates six electrons.

The sandwich complex $\mathrm{Cr}({\eta }^5\text{-}{\mathrm{C}}_5{\mathrm{H}}_5{)}_2$, with a conventional 16-electron count.

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.

The cation $[\mathrm{Co}({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5{)}_2{]}^{+}$, conventionally Co(III), $d^6$, and 18-electron.

The neutral sandwich compound $\mathrm{Co}({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5{)}_2$, conventionally Co(II), $d^7$, with 19 valence electrons. One-electron oxidation gives the 18-electron cobaltocenium cation.

The sandwich compound $\mathrm{Fe}({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5{)}_2$, conventionally Fe(II), $d^6$, with 18 valence electrons.