Nitrogen pentafluoride

Nitrogen pentafluoride is a theoretical compound of nitrogen and fluorine with the chemical formula NF5. It is hypothesized to exist based on the existence of the pentafluorides of the atoms below nitrogen in the periodic table, such as phosphorus pentafluoride. Theoretical models of the nitrogen pentafluoride molecule are either a trigonal bipyramidal covalently bound molecule with symmetry group D3h, or [NF4]+F−, which would be an ionic solid.

Ionic solid
A variety of other tetrafluoroammonium salts are known ([NF4]+X−), as are fluoride salts of other ammonium cations ([NR4]+F−).

In 1966, W. E. Tolberg first synthesized a five-valent nitrogen compound of nitrogen and fluorine when tetrafluoroammonium compounds, tetrafluoroammonium hexafluoroantimonate [NF4]+[SbF6]− and tetrafluoroammonium hexafluoroarsenate [NF4]+[AsF6]− were made. In 1971 C. T. Goetschel announced the preparation of [NF4]+[BF4]− and also produced a white solid assumed to be tetrafluoroammonium fluoride ([NF4]+F−). This was made by treating nitrogen trifluoride and fluorine with 3 MeV electron radiation at 77 K. It decomposed above 143 K back into those ingredients. Theoretical studies also show the ionic compound is very likely to decompose to nitrogen trifluoride and fluorine gas.

Karl O. Christe synthesised bis(tetrafluoroammonium) hexafluoronickelate ([NF4]+)2[NiF6](2−). He also prepared compounds with manganese, a fluorouranate, a perchlorate, a fluorosulfate and [N2F3]+ salts. Christe attempted to make [NF4]+F− by metathesis of [NF4]+[SbF6]− with CsF in HF solvent at 20 °C. However, a variant, tetrafluoroammonium bifluoride ([NF4]+[HF2]−*nHF), was produced. At room temperature it was a milky liquid, but when cooled, turned pasty. At −45 °C it had the form of a white solid. When reheated it frothed, giving off F2, HF and NF3 as gases. This has CAS number 71485-49-9.

I. J. Solomon believed that nitrogen pentafluoride was produced by the thermal decomposition of [NF4]+[AsF6]−, but experimental results were not reproduced.

Dominik Kurzydłowski and Patryk Zaleski-Ejgierd predict that a mixture of fluorine and nitrogen trifluoride under pressure between 10 and 33 GPa forms [NF4]+F− with space group R3m. This is a high-pressure oxidation. Over 33 GPa it will form a stable ionic compound with formula ([NF4]+)2[NF6]−F– with space group I4/m. Over 151 GPa this is predicted to transform to [NF4]+[NF6]− with space group P4/n. A NF5 molecular compound is not stable under any pressure conditions.

Covalent molecule
For a NF5 molecule to form, five fluorine atoms have to be arranged around a nitrogen atom. There is insufficient space to do this at typical nitrogen–fluorine covalent-bond lengths, so at least some bonds are forced to be longer. Calculations show that fragmentation to form NF4 and F radicals would have a transition state barrier of around 15.8-20.0 kcal/mol and that this process is thermodynamically favourable (exothermic) by 9 kcal/mol. Nitrogen pentafluoride also violates the octet rule in which compounds with eight outer shell electrons are particularly stable.