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.