How to think about oxygen at a crystal site with formal charge 3+?

Question

I am building a model of simple crystal lattices of rock salt type for different compounds. My goal is to make use of the measured values of ionization energies of atoms and formation energies of compounds from standard state elements and atomization energies of the elements to gain an understanding of the bond types.

The formal charges required for theoretical reasons are, in some cases, not the same as the charges of ions in reality. While looking at a table of Shannon radii, I noticed, it contains values for six-coordinated $\ce{N^5+}$, among other seemingly extreme charges. However, there is no six-coordinated $\ce{O^3+}$.

• Is there really a compound, where nitrogen is six-coordinated 5+ cation? If not, how was this value calculated?
• Can I reliably calculate crystal radii for any element in any formal oxidation state? I only need positive ions' radii.
• Maybe I should, in case of $\ce{O^3+}$, try another approach and assign a minimum possible radius based on Pauling's first rule . For example: if the measured bond length is $\pu{209 pm}$, then the theoretical $\ce{O^3+}$ radius will be $$\frac{0.414}{1+0.414}\times\pu{209 pm} = \pu{61 pm}$$ for octahedral coordination. Is it safe to assume the real radius is less than that?

Answer 1

Of the two species mentioned in the question, hexacoordinate nitrogen(V) (the Roman numeral V means oxidation state, not formal charge) is the better established case in prospective crystals. No compound having this nitrogen state is known experimentally, but theoretical calculations predict a nitrogen-pentafluoride salt containing $\ce{NF6^-}$ at high pressure[1]. The proposed hexacoordinate nitrogen(V) radius is likely based on this calculation.

Pentacoordinate nitrogen, in a lower oxidation state, has been experimentally obtained in an organic complex[2].

References

1. Kurzydłowski, D., Zaleski-Ejgierd, P. "Hexacoordinated nitrogen(V) stabilized by high pressure". Sci Rep 6, 36049 (2016). https://doi.org/10.1038/srep36049

2. Chenting Yan, ORCID logo a Masato Takeshita,a Jun-ya Nakatsuji,a Akihiro Kurosaki, Kaoko Sato,Rong Shang, Masaaki Nakamoto Yohsuke Yamamoto, Yohei Adachi, Ko Furukawa, Ryohei Kishid and Masayoshi Nakano. "Synthesis and properties of hypervalent electron-rich pentacoordinate nitrogen compounds". Chem. Sci., 2020, 11, 5082-5088. https://doi.org/10.1039/D0SC00002G.