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In my first high school chemistry class, I misremembered nitrate as being $\ce{NH2-}$, rather than $\ce{NO3-}$, and wrote down a formula for "ammonium nitrate" that was $\ce{NH4NH2}$ (rather than $\ce{NH4NO3}$). Now I'm curious if that could actually exist.

I tried seeing if $\ce{NH2-}$ could exist as a polyatomic ion, and it seemed like it should, with the hydrogens being single bonds and the nitrogen having two lone pairs.I looked around for a while, and found azanide, which seems to look how I expected it to. I can find a few examples of it forming ionic bonds with alkali metals (where it's confusingly called "amide"), like lithium amide, and the article mentions silver(I) amide, but I still couldn't find anything by searching for "ammonium azanide" or "ammonium amide".

Is there some reason why $\ce{NH4NH2}$/ammonium azanide couldn't exist, or has it just not been looked for? Is there a way to predict what it would be like?

(I'm still in high school chemistry, so if I'm missing anything obvious that's probably why :p)

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    $\begingroup$ $\ce{NH2-}$ is very strong base, much stronger than $\ce{OH-}$, and would react with the weak acid $\ce{NH4+}$ forming $\ce{NH3}$. $\endgroup$
    – Poutnik
    Commented Mar 6 at 21:20
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    $\begingroup$ @Poutnik This would make a great answer $\endgroup$
    – Radvylf Programs
    Commented Mar 6 at 21:34
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    $\begingroup$ Not so great. That's the obvious thing, if you knew their acid-base properties. Still, many strange things can happen, if you try hard enough. Water can be fully ionic, why not ammonia? But your thing is still considered ammonia, no matter how far equilibrium would be shifted - that's perhaps both a strength and a weakness of chemistry. $\endgroup$
    – Mithoron
    Commented Mar 6 at 22:10
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    $\begingroup$ I have intentionally limited my comment to a simple, high school level. Answers to questions about existence of a specific compounds often depends on conditions. Many compounds generally assumed not to exist are realized to exist if conditions are extreme enough. Typical scenarios are cryogenic conditions in inert solid matrix, extreme pressures or interstellar space. $\endgroup$
    – Poutnik
    Commented Mar 7 at 9:15

2 Answers 2

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Technically, ordinary liquid ammonia may be regarded as a very dilute solution of ammonium azanide through its autoionization. (Water is similarly very dilute hydronium hydroxide.)

This technicality aside, we can indeed have ammonia convert to an ionic solid form $\ce{NH4^+NH2^-}$. Ninet et al1 calculated that under 150 GPa pressure at sufficiently low temperature solid ammonia switches from a molecular form to the ionic one referenced above, adopting two different ionic crystal lattice structures. Experimental evidence for this transformation was obtained from high-pressure IR spectroscopy showing the presence of $\ce{NH4^+}$ at 150 GPa. The figure below, taken originally from 1, shows the ionic structures and the temperature and pressure range for them on the lower right. Conditions suitable for forming such ionic phases (with ammonia alone or with ammonia and water together) may exist in the interiors of Uranus and Neptune.

phase diagram of NH3

Reference

  1. Ninet, S., Datchi, F., Dumas, P., Mezouar, M., Garbarino, G., Mafety, A., Pickard, C. J., Needs, R.J., Saitta, A.M. "Experimental and theoretical evidence for an ionic crystal of ammonia at high pressure" Physical Review B., 89(17): art.n° 174103. (2014).
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In standard conditions ammonium amide in pure form wouldn't be stable as it would turn into ammonia through proton transfer.

However, it is being taught that in liquid ammonia there is an equilibrium of self-ionisation present, similiar as in water, and that would result in extremally small amounts of ammonium amide in liquid ammonia.

Interestingly, there has been a theoretical study showing that solid ammonia under very high pressure could have a ionic (ammonium amide) structure: https://www.nature.com/articles/nmat2261

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