# Can H4O 2+ form?

Like $\ce{NH4+}$ ,Is there any possibility of formation of $\ce{H4O^{2+}}$ (of tetrahedral structure)?

My theory is: it can be formed by osmosis setup where heavy acids like $\ce{H3PO4}$ or $\ce{H2S2O7}$ are passed through a semipermeable membrane like gelatinous copper ferrocyanide SPM, which allow only small ions like H+ to pass.

This then could form $\ce{H3O+}$, then its lone pair could attack the excess $\ce{H+}$ in solution to form forming $\ce{H4O^{2+}}$.

• Semipermeable membranes just don't work like that. You can't separate cations from anions this way, and that's a very, very huge "can't" (as in "a mosquito can't lift the Empire State Building"). The possible (?) existence of $\ce{H4O^2+}$ in extreme conditions is another story. – Ivan Neretin Apr 9 '16 at 6:55
• – Mithoron Apr 9 '16 at 11:14
• Osmosis can reject anions as stated in rejection of anions by osmosis @IvanNeretin – Swastik Apr 10 '16 at 7:46
• Sure, but it would also place another ions instead of them. – Ivan Neretin Apr 10 '16 at 8:48

The existence of $\ce{H4O^{2+}}$ has been inferred from hydrogen/deuterium isotopic exchange monitored through $\ce{^{17}O}$ NMR spectroscopy in the most extremely acidic condensed phase superacid we can make, fluoroantimonic acid ($\ce{HF:SbF5}$ or $\ce{HSbF6}$). It seems that even the slightly weaker but still very much superacidic magic acid $\ce{HSO3F:SbF5}$ is not strong enough to effect measurable protonation of $\ce{H3O+}$ into $\ce{H4O^{2+}}$. It likely also can exist in the gas phase by exposing water to bare protons, and might have some importance in astrochemistry. Relatedly, $\ce{H4S^{2+}}$ also has been inferred to exist through the same type of measurements, and is easier to make, existing in magic acid mixtures.
A nice source for your superacidic needs is books by George Olah, the father of superacidic chemistry. In particular, Onium Ions has a section on the protohydronium dication $\ce{H4O^{2+}}$ on page 435. There are many other curious cations analyzed such as $\ce{CH6^{2+}}$ and even $\ce{CH7^{3+}}$. When it comes to superacids, nothing is sacred!
• Minor point. Fluoroantimonic acid is not $\ce{HSbF6}$. The fluoroantimonate anion is such a weak proton acceptor that it never gets protonated at all; therefore the acid forms only ionic species. See here for a description of these ionic species. – Oscar Lanzi Apr 4 at 23:32