Oxofluorides and oxochlorides of nitrogen

   Several oxofluorides and oxochlorides of nitrogen are known, but all are unstable gases or volatile liquids which are rapidly hydrolysed. Nitrosyl halides FNO, ClNO and BrNO are formed in reactions of NO with F₂, Cl₂ and Br₂ respectively; structural details for gas-phase molecules are shown in above. The short N^_O bond lengths indicate triple rather than double bond character and a contribution from the left-hand resonance structure in the resonance pair below is clearly important. Crystals of FNO and ClNO have been grown from condensed samples of the compounds, and their solid state structures have been determined at 128 and 153 K, respectively. Compared with those in the gas phase, FNO molecules in the crystal have shorter (108 pm) N^_O and longer (165 pm) N^_F bonds. A similar trend is seen

for ClNO (solid: N^_O =105 pm, N^_Cl = 219 pm). These data suggest that the [NO]‏X^- form becomes more dominant in resonance pair 14.24 on going from gaseous to solid XNO.

    Nitryl fluoride, FNO₂, and nitryl chloride, ClNO₂, are prepared, respectively, by fluorination of N₂O₄  and oxidation of ClNO (using e.g. Cl₂O or O₃).   Both are planar molecules; FNO₂ is isoelectronic with [NO₃]^-.

   The oxohalides FNO, ClNO, FNO₂ and ClNO₂ combine with suitable fluorides or chlorides to give salts containing [NO]‏ or [NO₂]‏, e.g. reactions 14.68–14.70. The complex fluorides may also be conveniently prepared in liquid BrF3.

   The main factor involved in the change from covalent to ionic halide is believed to be the enthalpy change accompanying the attachment of the halide ion to the halide acceptor.

   The reaction of FNO with the powerful fluorinating agent IrF₆ results in the formation of the nitrogen(V) oxofluoride F₃NO. Above 520 K, F₃NO is in equilibrium with FNO and F₂. Resonance structures below can be written to depict the bonding, and the short N^_O bond (116 pm) and long N^_F bonds (143 pm) suggest that contributions from below (and similar structures) are important.

   Reactions of F₃NO with strong F^- acceptors such as BF₃ and AsF₅ yield the salts [F₂NO]‏⁺[BF₄]^- and [F₂NO]‏⁺[AsF₆]^-. In the [AsF₆]^-  the [F₂NO]‏ ion is planar (structure right above), consistent with the formation of an N(2p)‑O(2p) π-bond (diagram below).