Nitrogen pentafluoride

Nitrogen pentafluoride (NF₅) is a theoretical compound of nitrogen and fluorine that 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 D_3h, or NF⁺
₄F⁻, 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 (NR⁺
₄F⁻).

In 1966, W. E. Tolberg first synthesized a five-valent nitrogen compound of nitrogen and fluorine when tetrafluoroammonium compounds, tetrafluoroammonium hexafluoroantimonate NF₄SbF₆ and tetrafluoroammonium hexafluoroarsenate NF₄AsF₆ were made.^[2] In 1971 C. T. Goetschel announced the preparation of NF₄BF₄ and also produced a white solid assumed to be tetrafluoroammonium fluoride (NF⁺
₄F⁻). 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.^[2] Theoretical studies also show the ionic compound is very likely to decompose to nitrogen trifluoride and fluorine gas.^[3]

Karl O. Christe synthesised bis(tetrafluoroammonium) hexafluoronickelate (NF₄)₂NiF₆.^[4] He also prepared compounds with manganese, a fluorouranate, a perchlorate, a fluorosulfate and N₂F⁺
₃ salts.^[5] Christe attempted to make NF₄F by metathesis of NF₄SbF₆ with CsF in HF solvent at 20 °C. However, a variant, tetrafluoroammonium bifluoride (NF₄HF₂·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.^[5] This has CAS number 71485-49-9.^[6]

I. J. Solomon believed that nitrogen pentafluoride was produced by the thermal decomposition of NF₄AsF₆, but experimental results were not reproduced.^[7]

Dominik Kurzydłowski and Patryk Zaleski-Ejgierd predict that a mixture of fluorine and nitrogen trifluoride under pressure between 10 and 33 GPa forms NF⁺
₄F⁻ with space group R3m. This is a high-pressure oxidation. Over 33 GPa it will form a stable ionic compound with formula (NF⁺
₄)₂NF⁻
₆F^– with space group I4/m. Over 151 GPa this is predicted to transform to NF⁺
₄NF⁻
₆ with space group P4/n.^[8] A NF₅ molecular compound is not stable under any pressure conditions.

• 
  NF₄⁺F⁻ R3m structure
• 
  (NF₄⁺)₂NF₆⁻F⁻ I4/m structure
• 
  NF₄⁺NF₆⁻ P4/n structure

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 66–84 kJ/mol (15.8–20.0 kcal/mol) and that this process is thermodynamically favourable (exothermic) by 38 kJ/mol (9 kcal/mol).^[9] Nitrogen pentafluoride also violates the octet rule in which compounds with eight outer shell electrons are particularly stable.^[10]

References