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Wang and Islam [1] state that the ferrocene molecule's band gap, i.e. the HOMO-LUMO gap at the Kohn-Sham level, is 5.03 eV, which corresponds to an absorption wavelength of at most 240 nm.

So, ferrocene should be colorless — except that it isn't. I found out that liquid water, which has an even higher band gap (7 eV) than ferrocene, is still colored because of vibronic, and not electronic, excitations.

Does the same hold for ferrocene as well, and, if it does, do the expected effects to the HOMO-LUMO gap of e.g. π-electron withdrawing/donating substituents (which would change the color of the parent molecule, were that color be dependent on the gap — again, except that it isn't) “change” the color in the “expected” ways?

P.S. Please read my long string of comments, involving explicit calculations done for the ionisation potential/negated Kohn-Sham HOMO energy of the hydride anion, on Oscar Lanzi's answer before adding more answers.

Reference

  1. Wang, F.; Islam, S. Impact of ionization of ferrocene: EOES of α- and β-electrons and the fingerprint orbital 8a₁’ of ferrocenium. 2015. DOI: 10.48550/ARXIV.1509.05122.
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    $\begingroup$ Comparing the color of water with that of ferrocene is like comparing the atmosphere of the Moon with that of Earth. Technically the Moon has an atmosphere, but by standards we typically work with on Earth -- really? $\endgroup$
    – Oscar Lanzi
    Jul 4 at 18:23
  • $\begingroup$ Well, there's obviously some electron transition in there, other than what you mention. I don't think that's a right paper to tell the actual situation, better check out something about UV-Vis. $\endgroup$
    – Mithoron
    Jul 5 at 14:14
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    $\begingroup$ Please do not refer to any comments. Edit your question as a single concise post. This is not a discussion forum and comments are only here to improve on existing posts. Plus, they are gone now. $\endgroup$
    – Martin - マーチン ♦
    Jul 5 at 20:59
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    $\begingroup$ All of the traditional DFT is Kohn-Sham, that really does not specify any level of theory. That also does not provide any insight in which way they have confirmed that they are not using an outlier functional. DFT isn't known for this kind of accuracy and I expect a whole range of HOMO LUMO gap values for different functionals. A good paper would review these, and calibrate, and discuss. $\endgroup$
    – Martin - マーチン ♦
    Jul 5 at 21:06
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    $\begingroup$ They've used b3lyp with a split valence basis set because that reproduced the ionisation potential. And that was their only concern. This level of theory is known to be right usually for the wrong reasons, error compensation and the lot. Also, they've used Koopmans' theorem, which in itself only works because of error compensation. Funny enough, they based parts of their argument on Hartree-Fock. They also don't discuss spin contamination (or I missed this). I have to be honest though, I didn't read the whole paper, it was too painful to follow. $\endgroup$
    – Martin - マーチン ♦
    Jul 5 at 21:37

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