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Carbon monoxide is not protonatable in aqueous solution; a quick Google search even leads to a paper that talks about using carborane acids, i.e. extreme superacids, to protonate the molecule.

However, there do not seem to be any reason for carbon monoxide to be non-protonatable in aqueous solution- for one thing, its HOMO is localised on the carbon, like the so-called "persistent" carbenes, and the latter are usually just too strong to exist unprotonated even in DMSO, let alone in water. Another problem is that other carbon-centered lone pairs are extremely strong bases as well(e.g. the methyl anion), unless it participates in resonance involving strongly electron-withdrawing groups(such as nitrile or triflyl- even fluorine fails), and I'm not really sure that the small oxygen-centered lobe of the HOMO of carbon monoxide (which is of charge-shift character) counts as "strongly electron-withdrawing".

My main question now follows- what is the reason for the extreme stability against protonation in aqueous solution of the carbon monoxide molecule? Is it simply because of its HOMO's low energy of -14eV (quoting NIST) or is it something else?

P.S. I'm NOT talking about the Lewis basicity of carbon monoxide here- the very reason that it easily complexes with transition metals is that transition metals have their own valence lone pairs to back-donate to the LUMO of carbon monoxide; this cannot happen with the proton, since the proton doesn't even have any electrons to start with, let alone valence lone pairs.

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  • $\begingroup$ does HOMO mean highest occupied molecular orbital? $\endgroup$
    – DialFrost
    Commented Mar 29, 2022 at 12:09
  • $\begingroup$ @DialFrost Yes, it is. $\endgroup$ Commented Mar 29, 2022 at 13:50
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    $\begingroup$ I think you're somewhat overestimating its low basicity. IIRC one can formylate aromatic compounds using CO and strong acid. If you wanted complete protonation, indeed something like carborane may be needed, though. $\endgroup$
    – Mithoron
    Commented Mar 29, 2022 at 14:00
  • $\begingroup$ Not relevant to aqueous solution but maybe of tangential interest- the HCO+ ion is “often the most abundant cation in molecular clouds” frontiersin.org/articles/10.3389/fspas.2021.655405/full $\endgroup$
    – Andrew
    Commented Mar 30, 2022 at 0:59

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