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Chemistry · Glossary

What is Metallocenes?

Also known as: sandwich compound · metallocene

Definition 20.10 University Chemistry — Year 3 · Chapter 20 — Organometallic Chemistry: Bonding and Ligands

A sandwich compound has a metal between two parallel planar ring ligands bonded through all their carbons; a metallocene is a sandwich compound with two η5\eta^5-cyclopentadienyl rings, M(CX5HX5)X2\ce{M(C5H5)2}.

Left: the sandwich structure of ferrocene (eclipsed rings). Right: the d-based frontier orbitals of metallocenes, e_2g and a_1g nearly non-bonding, e_1g antibonding, filled for the ferrocenium cation (17 electrons), ferrocene (18), cobaltocene (19) and nickelocene (20, one electron in each e_1g orbital, spins parallel by Hund’s rule).
Left: the sandwich structure of ferrocene (eclipsed rings). Right: the dd-based frontier orbitals of metallocenes, e2ge_{2g} and a1ga_{1g} nearly non-bonding, e1g∗e_{1g}^\ast antibonding, filled for the ferrocenium cation (17 electrons), ferrocene (18), cobaltocene (19) and nickelocene (20, one electron in each e1g∗e_{1g}^\ast orbital, spins parallel by Hund’s rule).
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