# Nature of a1g Molecular Orbital Interactions in Ferrocene

While learning about ferrocene I've come across this molecular orbital diagram. What particularly confused me is the nature of the a'1g orbital. My professor taught that a'1g and e2g are considered HOMO bonding orbitals, but this MO diagram clearly shows the a'1g as an MO with antibonding character.

To make sure it wasn't just a bad MO diagram, I looked for other MO diagrams, but they consistently display a'1g as a higher energy orbital compared to Iron's $$3dz^2$$ orbital.[a][b] So, slight anti-bonding character, right?

The problem is, looking at bond-length comparison, the a'1g orbital is slightly bonding. The Fe-Cp distance for ferrocene is 1.66 Å; for ferrocenium, it's 1.68 Å.[c] Given that the removal of an electron from the HOMO a'1g orbital resulted in the lengthening of Fe-Cp distance, it should have bonding character.

So what is going on here? Why is there this discrepancy? How are these orbitals actually interacting with each other?

• HOMO stands for highest occupied molecular orbital. Of the nine occupied MOs in that diagram, the one that is highest in energy is a'1g. Your assumption that the HOMO cannot be antibonding is not correct.
– Zhe
Jan 16 at 20:08
• @Zhe I understand that. My question is about the seeming discrepancy between its position on the MO diagram (showing antibonding character) and physical analyses (Fe-Cp distance comparison showing bonding character). Jan 17 at 3:03
• But ferrocenium is charged. What's more important in determining structure, changes in electronic structure or charge/charge repulsion?
– Zhe
Jan 17 at 16:54
• @Zhe If anything, I would think ferrocenium should have greater coulombic attraction with the negatively-charged cyclopentadienyl ligands and result in shorter bond lengths. Is that wrong? Jan 17 at 18:42
• My intuition says it's more complicated than that. You're assuming that even in the complex, the Cp's are strongly charged negative and the positive is more localized to the Fe center, and I'm not convinced that that's the case.
– Zhe
Jan 17 at 20:03