Tetrafluoroammonium

The tetrafluoroammonium cation (also known as perfluoroammonium) is a positively charged polyatomic ion with chemical formula NF4+. It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine. Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF4, trifluoramine oxide ONF3 and the tetrafluoroborate BF4- anion. The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion (HF2-), tetrafluorobromate (BrF4-), metal pentafluorides (MF5- where M is Ge, Sn, or Ti), hexafluorides (MF6- where M is P, As, Sb, Bi, or Pt), heptafluorides (MF7- where M is W, U, or Xe), octafluorides (XeF82-), various oxyfluorides (MF5O- where M is W or U; fluorosulfuric acidFSO3-, BrF4O-), and perchlorate (ClO4-). Attempts to make the nitrate salt, NF4NO3, were unsuccessful because of quick fluorination: NF4+ + NO3- → NF3 + FONO2. The geometry of the tetrafluoroammonium ion is tetrahedral, with an estimated nitrogen-fluorine bond length of 124 pm. All fluorine atoms are in equivalent positions. Tetrafluoroammonium salts are prepared by oxidising nitrogen trifluoride with fluorine in the presence of a strong Lewis acid which acts as a fluoride ion acceptor. The original synthesis by Tolberg, Rewick, Stringham, and Hill in 1966 employs antimony pentafluoride as the Lewis acid: NF3 + F2 + SbF5 → NF4SbF6 The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C: NF3 + F2 + AsF5 → NF4AsF6 The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt: NF3 + F2 + BF3 → NF4BF4 NF4+ salts can also be prepared by fluorination of NF3 with krypton difluoride (KrF2) and fluorides of the form MFn, where M is Sb, Nb, Pt, Ti, or B. For example, reaction of NF3 with KrF2 and TiF4 yields [NF4+]2TiF62-. Many tetrafluoroammonium salts can be prepared with metathesis reactions. Tetrafluoroammonium salts are extremely hygroscopic.