Nitrogen pentafluoride
Nitrogen pentafluoride (NF) 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, or NFF, which would be an ionic solid. Ionic solid. A variety of other tetrafluoroammonium salts are known (NFX), as are fluoride salts of other ammonium cations ). In 1966, W. E. Tolberg first synthesized a five-valent nitrogen compound of nitrogen and fluorine  when tetrafluoroammonium compounds, tetrafluoroammonium hexafluoroantimonate NFSbF and tetrafluoroammonium hexafluoroarsenate NFAsF were made. In 1971 C. T. Goetschel announced the preparation of NFBF and also produced a white solid assumed to be tetrafluoroammonium fluoride (NFF). 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 (NF)NiF. He also prepared compounds with manganese, a fluorouranate, a perchlorate, a fluorosulfate and NF salts . Christe attempted to make NFF by metathesis of NFSbF with CsF in HF solvent at 20 °C. However, a variant, tetrafluoroammonium bifluoride (NFHF·"n"HF), 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 F, HF and NF as gases. This has CAS number 71485-49-9. I. J. Solomon believed that nitrogen pentafluoride was produced by the thermal decomposition of NFAsF, 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 NFF with space group "R3m". This is a high-pressure oxidation. Over 33 GPa it will form a stable ionic compound with formula (NF)NFF with space group "I4/m". Over 151 GPa this is predicted to transform to NFNF with space group "P4/n". A NF molecular compound is not stable under any pressure conditions. Covalent molecule. For a NF molecule to form, five fluorine atoms have to be arranged around a nitrogen atom. There is insufficient space to do this in the most compact  way, so that bond lengths are forced to be longer. Calculations show that the NF molecule is thermodynamically favourably inclined to form NF and F radicals with energy 36 kJ/mol and a transition barrier around 67–84 kJ/mol. Nitrogen pentafluoride also violates the octet rule in which compounds with eight outer shell electrons are particularly stable.