Nitrogen halides

    Nitrogen is restricted to an octet of valence electrons and does not form pentahalides. The fact that nitrogen pentahalides are not known has also been attributed to the steric crowding of five halogen atoms around the small N atom.

     Important nitrogen halides are NX₃ (X= F, Cl), N₂F₄ and N₂F₂, selected properties for which are listed in Table 1.1; NBr₃ and NI₃ exist but are less well characterized than NF₃ and NCl₃.

   Nitrogen trifluoride  which must be carried out in a controlled manner, or by electrolysis of anhydrous NH₄F/HF mixtures.

     NF₃ is the most stable of the trihalides of nitrogen, being the only one to have a negative value of Δ_fH^o (Table 1.1). It is a colourless gas which is resistant to attack by acids and alkalis, but is decomposed by sparking with H₂ . The resistance towards hydrolysis parallels that observed for the carbon tetrahalides.

 The gas-phase structure of NF₃ is trigonal pyramidal  and the molecular dipole moment is very small (Table 1.1). In contrast to NH₃ and PF₃, NF₃ shows no donor properties.

Nitrogen trichloride is an oily, yellow liquid at 289 K, but it is highly endothermic and dangerously explosive (Table1.1). The difference in stabilities of NF₃ and NCl₃ lies in the relative bond strengths of N^_F over N^_Cl, and of Cl₂ over F₂. .    

   Diluted with air, NCl₃ is used for bleaching flour since hydrolysis by moisture forms HOCl. Alkalis hydrolyse NCl₃ according to equation 1.16.

     Nitrogen tribromide is more reactive than NCl₃, and explodes at temperatures as low as 175 K.

   Nitrogen triiodide has been made by reacting IF with boron nitride in CFCl₃. Although NI₃ is stable at 77K and has been characterized by IR, Raman and ¹⁵N NMR spectroscopies, it is highly explosive at higher temperatures (Δ_fH^o (NI₃,g) = 287 kJ mol^-1). The reaction between concentrated aqueous NH3 and [I₃]^- yields NH₃ :NI₃, black crystals of which are dangerously explosive (Δ_fH^o(NH₃:NI₃,s) =  ‏146 kJ  mol^-1) as the compound decomposes to NH₃, N₂ and I₂.

   The nitrogen fluorides N₂F₄ and N₂F₂ can be obtained from reactions 1.18 and 1.19; properties of these fluorides are listed in Table 1.1, and both fluorides are explosive.

    The structure of N₂F₄ resembles that of hydrazine, except that both the gauche and trans (staggered) conformers are present in the liquid and gas phases. At temperatures above 298 K, N₂F₄ reversibly dissociates into blue NF₂^. radicals which undergo many interesting reactions.

 Dinitrogen difluoride, N₂F₂, exists in both the trans- and cis-forms (above), with the cis-isomer being thermodynamically the more stable of the two but also the more reactive. Reaction above gives a selective method of preparing trans-N₂F₂; isomerization by heating gives a mixture of isomers from which cis-N₂F₂ can be isolated by treatment with AsF₅ .   Mixture of isomers:

   Reaction 14.63 illustrates the ability of N₂F₂ to donate F^- to strong acceptors such as AsF₅ and SbF₅, a reaction type shared by N₂F₄ . The cation [NF₄]⁺‏ is formed in reaction below. We return to the properties of AsF₅ and SbF₅ later.