# Benzene as a pi-acceptor?

In coordination chemistry, we classify ligands as sigma donors, pi acceptors and donors. Benzene is a pi donor, as is the cyclopentadienyl anion, but do these two possess any pi acceptor character? (Can a ligand be both pi acceptor and pi donor?)

• Of course they do. Apr 27 '20 at 13:41
• Mithoron, won't the pi-acceptance hinder the aromatic stabilisation? The LUMO is antibonding, right? Apr 27 '20 at 15:31
• It's antibonding like, what, 90 % of all LUMOs ? Still, they're used for backbonding all the time. Apr 28 '20 at 18:19

Of course these interactions sacrifice aromaticity in the cyclopentadienyl rings, but it's a bargain with the bonds forming between carbon and the transition element. Aromaticity is not so much broken as it is superseded by a further bond deolcalization. In the especially well-known case of ferrocene, the energy bargain is especially good because when all is said and done with the iron/cyclopentadienyl electron exchange, you've exactly filled the bonding orbitals and left the antibonding orbitals empty. The diagram from this source shows the molecular orbital structure in the case of a $$3d$$ metal. For ferrocene where the iron(II) center provides six additional electron, they fill the orbitals in the box right up to the metal $$3d$$ orbital level (the HOMO is the $$a_{1g}$$ orbital inside the box) but not beyond. Note that the filled $$e_{2g}$$ orbital has a component from the antibonding orbitals of the cyclopentadienide, showing the pi acceptor action of the latter.
• So if there is both pi-acceptor and pi-donor capacity, what happens to the gap between $\ce{t_2_g}$ and $\ce{e_g^*}$ levels. Is it increased or decreased? May 6 '20 at 9:09
• Not octahedral, therefore we don't really have $t_{2g}$or $e_g$. See here for a proper description of the molecular orbitals of metallocenes. May 6 '20 at 9:24