# Why does N(OH)3 not exist?

The hydrolysis of Group-15 tetrahalides gives us their hydroxides via a $$\ce{S_{N^2}}$$ mechanism: $$\ce{PCl3 + 3H2O -> H3PO3 + 3HCl}$$ Nitrogen Trichloride, however, yields a different set of products, not forming the trihydroxide as expected. $$\ce{NCl3 + 3H2O -> NH3 + 3HOCl}$$

A little bit of research led me to this exact same question (with 2 unsatisfactory answers) and this article, which claims that it exists, but I cannot read it. Why does $$\ce{N(OH)3}$$ not exist?

Another thought along similar lines is the existence of gem-diols in general, which are quite unstable, as well as $$\ce{C(OH)4}$$, which does not seem to exist as well. Is this an abnormal property of period 2 elements? All the other elements in group 14 and 15 form stable hydroxides whereas the first members do not seem to do so.

• You are confusing two entirely different and unrelated questions. The direction of hydrolysis is determined by the difference in electronegativity. Indeed, why the hydrolysis of PCl3 does not end up in PH3 and HOCl? This is not because PH3 or HOCl doesn't exist; in fact they both do. Then why? – Ivan Neretin Feb 4 at 9:30
• When you say that a molecule like $\ce{N(OH)3}$ does not exist, you must specify under what conditions too. $\ce{N(OH)3}$ certainly doesn't exist in aqueous solution, but in free space it probably does. – MaxW Feb 4 at 10:45
• What you render as $\ce{H3PO3}$ is not predominantly $\ce{P(OH)3}$ either. In Group 15 you have to go to arsenic to get predominantly the hydroxide under normal laboratory conditions. – Oscar Lanzi Feb 4 at 11:12
• Even if you could make it, wouldn't it just decompose into nitrous acid and water? I haven't done the math, but I'd expect the decomposition to be both energetically and entropically favorable, and I doubt the kinetic barrier is high enough to prevent it at normal laboratory temperatures. – Ilmari Karonen Feb 4 at 19:21
• @undefined I assume you meant Na3NO3. Anyways, there is no such $\ce{{NO_3}^{3-}}$ ion. The Na3NO3 we are seeing is an oxide-nitrate salt $\ce{Na3[NO2-]O}$ containing nitrite ion. See this article: onlinelibrary.wiley.com/doi/epdf/10.1002/anie.197603762 – Nilay Ghosh Feb 6 at 16:24

Well let's not focus on your text but on the actual question here about $$\ce{N(OH)3}$$. I can only give you some ideas here but not a definite answer.

Whenever we have tests for our first semester students there are some who forgot what a nitrate was, or nitric acid. But as we ask for a nitrate as trivial name an ortho-nitrate is an accepted answer as well. And many people will just try to balance things with $$\ce{X(OH)_n}$$ until it fits. It's not wrong in a test but we have to think about these compounds.

Let's take silicates. There are ortho-silicates, so salts that derive from the ortho-silicic acid $$\ce{H4SiO4}$$ or $$\ce{Si(OH)4}$$. If you try to make this acid however it will start to polymerize while water is being produced. The reason for this is that you have a positively charged $$\ce{H^+}$$ that can be deprotonated easily close to an $$\ce{Si^4+}$$ center in your $$\ce{Si-OH}$$-bond. Hence two of these centers will react to something like $$\ce{Si-O-Si + H2O}$$ until something like a meta-silicic acid forms.

Now going back to our nitric acid first with $$\ce{N^{+5}}$$. We know the acid is called $$\ce{HNO3}$$. If we add a water to this we will get the ortho-nitric acid $$\ce{NO(OH)3}$$ or $$\ce{H3NO4}$$. Much as with the ortho-silicates we can find examples like the compound $$\ce{Na3NO4}$$ which could formally be the salt of the ortho-nitric acid.

Now if we want the same for nitrous acid $$\ce{HNO2}$$, well let's add water $$\ce{H3NO3}$$ or $$\ce{N(OH)3}$$. Does it exist? Well there is a salt $$\ce{Na3NO3}$$. The problem is, this is no ortho-nitrite but an oxide-nitrite $$\ce{Na3[NO2]O}$$.

I can't tell you however why there hasn't been an ortho-nitrite formed, yet. These ortho-acids are just not very stable.

Although the acid and its salts are unknown, orthonitrite esters have been found, and from a natural source at that [1]. The orthonitrite function, which is trivalent rather than monovalent as nitrate would be, is incorporated into a macrocycle that hinders its decomposition.

The picture below is from Ref. 1.

Reference:

1. Leanne Murray,* Tang K. Lim,* Graeme CurrieB and Robert J., "Aplidites (A-G): Macrocyclic Orthonitrites from an Australian Tunicate, Aplidium sp.", Aust. J. Chem., 1995, 48, 1253-1266.Link

• There are several dozens of structurally characterized μ₃-nitrato metal complexes, so probably there is no need to go that exotic:) – andselisk Feb 4 at 14:08
• Maybe the nitrato complexes are the more exotic ones depending on your point of view. The reference says the orthonitrite function "has not been previously described", perhaps meaning "...in organic chemistry". – Oscar Lanzi Feb 4 at 14:12
• @andselisk can you provide amy reference(s)? Google searching of the phrase "mu3-nitrato complexes" does not work for me. Thanks. – Oscar Lanzi Feb 5 at 12:13
• – andselisk Feb 6 at 3:32
• Thank you @andselisk. When I apportion charges with normal or article-specified metal oxidation stares I find each $\ce{NO3}$ moiety has only $-1$ charge instead of the hoped for $-3$. It's as if one covalent bond and two ionic ones were delocalized over the three linkages (like the $\ce{S-F}$ bonds in $\ce{SF6}$), versus three separare covalent linkages as in the orthonitrite esters/alkaloids of my Ref. [1]. – Oscar Lanzi Feb 6 at 11:09

The question presupposes that $$\ce{N(OH)3}$$ does not exist.

However, Ab initio molecular dynamics evidence of a new stable symmetric Cs structure for N(OH)3 Chemical Physics Letters volume 435, pages 34-38 says:

We point out that very few studies have addressed N(OH)3 and this study is relevant in the context of its possible detection in the gas phase.

See also A new non-symmetric N(OH)3 species: Comparison with the C3 species and thermochemistry at the HF, DFT, MP2, MP4 and CCSD(T) levels of theory Journal of Molecular Structure: THEOCHEM, volume 802, pages 111-115.

• Did yhey consider tautomerization like phosphorous/phosphonic acid? – Oscar Lanzi Feb 5 at 21:44
• @OscarLanzi no, I read the full text of both articles and they don't mention anything where an H is bonded to the N. Only three slightly different N(OH)3 species, one with $C_3$ symmetry, one $C_1$,and one $C_s$. The J. Molecular Structure article simulates IR spectra, saying "...can perhaps be used as reference data to compare with experimental IR results obtained in the gas phase in the near future". – DavePhD Feb 6 at 14:21

It isn't just nitrogen that fails to form an orthoacid with the formula $$\ce{X(OH)3}$$. The same thing happens with phosphorus. The phosphorous acid $$\ce{H3PO3}$$ is not $$\ce{P(OH)3}$$, with three equivalent $$\ce{OH}$$ groups as you might believe. This phosphorous acid has one H atom that is impossible to neutralize. Its structure is better described by $$\ce{HPO(OH)2}$$, with a central $$\ce{P}$$ atom surrounded by one $$\ce{H}$$, one $$\ce{O}$$ and two $$\ce{OH}$$ groups. Neutralized by a hydroxide it can produce two series of salts, and not three. With $$\ce{NaOH}$$, it gives: $$\ce{NaH2PO3}$$, $$\ce{Na2HPO3}$$. $$\ce{Na3PO3}$$ does not exist.

Arsenic, the next element in Group 15, does form the acid $$\ce{H3AsO3}$$ in water solution, and normal salts thereof.

Going across from left to right on period 2 and 3, the likelihood of forming stable hydroxides seems to go down. Lithium hydroxide, boron hydroxide, indium hydroxide, they all exhibit pretty good stability. As atomic radius decreases, the electronegative oxygens can be expected to drawn closer. Even nitric acid utilizes resonance to maintain stability.

This question has not been answered formally by modern research.