Choose the most appropriate Lewis structure for $\ce{NH3O}$ among the two:

NH2-OH and enter image description here

The oxygen in the second case has 3 nonbonding pairs around it.

I chose the first as an answer, yet the second seems to be the correct one. Can anyone please explain why?


Technically, both the structures $\ce{H2N-OH}$ and $\ce{H3N\bond{->}O}$ may exist. However, in reality hydrogen atom is rather prone to migration and the second structure is not favorable. So, for a compound with composition $\ce{NH3O}$, the correct structure would be $\ce{H2N-OH}$. The structure of second type is stable for compound $\ce{NOF3}$ and may be observed for amine oxides like $\ce{(C2H5)3N\bond{->}O}$ or pyridine N-oxide.

Why $\ce{H2N-OH}$ and not $\ce{H3N\bond{->}O}$? One can come with several explanations for that, but I would like to focus on one: charge distribution. In $\ce{H3N\bond{->}O}$, the nitrogen has to carry a formal positive (and actually very real) charge, while for $\ce{H2N-OH}$, the structure has no formal charges. Still, according to some sources[1] up to 20% of hydroxylamine in water exists as ammonia oxide $\ce{H3N\bond{->}O}$, probably, due to stabilization by hydrogen bonds.

Somewhat similar uncertainty may be found for phosphorus compounds (like hypophosphorous acid) and sulfur compounds (like sulfinic acids).

  1. Kirby, A. J.; Davies, J. E.; Fox, D. J.; Hodgson, D. R. W.; Goeta, A. E.; Lima, M. F.; Priebe, J. P.; Santaballa, J. A.; Nome, F. Ammonia oxide makes up some 20% of an aqueous solution of hydroxylamine. Chem. Commun. 2010, 46 (8), 1302. DOI: 10.1039/b923742a.

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