Nitrides, phosphides, arsenides,antimonides and bismuthides Nitrides

  Classifying nitrides is not simple, but nearly all nitrides fall into one of the following groups, although, as we have seen for the borides and carbides, some care is needed in attempting to generalize:

• saline nitrides of the group 1 and 2 metals, and aluminium;
• covalently bonded nitrides of the p-block elements .
• interstitial nitrides of d-block metals;
• pernitrides of the group 2 metals.

    The classification of ‘saline nitride’ implies the presence of the N^3- ion  this is unlikely. However, it is usual to consider Li₃N, Na₃N, Be₃N₂, Mg₃N₂, Ca₃N₂, Ba₃N₂ and AlN in terms of ionic formulations. Hydrolysis of saline nitrides liberates NH₃. Sodium nitride is very hygroscopic, and samples are often contaminated with NaOH.

    Among the nitrides of the p-block elements, Sn₃N₄ and the γ-phase of Si₃N₄ represent the first examples of spinel nitrides.    Nitrides of the d-block metals are hard, inert solids which resemble metals in appearance, and have high melting points and electrical conductivities. They can be prepared from the metal or metal hydride with N₂ or NH₃ at high temperatures. Most possess structures in which the nitrogen atoms occupy octahedral holes in a close-packed metal lattice. Full occupancy of these holes leads to the stoichiometry MN (e.g. TiN, ZrN, HfN, VN, NbN); cubic close-packing of the metal atoms and an NaCl lattice for the nitride MN is favoured for metals in the earliest groups of the d-block.   Pernitrides contain the [N₂]^2- ion and are known for barium and strontium. BaN₂ is prepared from the elements under a 5600 bar pressure of N₂ at 920K.

    It is structurally related to the carbide ThC₂ and contains  isolated [N₂]^2- ions with an N^_N distance of 122 pm, consistent with an N=N bond. The strontium nitrides SrN₂ and SrN are made from Sr₂N at 920K under N₂ pressures of 400 and 5500 bar, respectively. The structure of SrN₂ is derived from the layered structure of Sr₂N by having half of the octahedral holes between the layers occupied by [N₂]^2- ions. Its formation can be considered in terms of N₂ (at high pressure) oxidizing Sr from a formal oxidation state of 1.5 to ‏2, and concomitant reduction of N₂ to [N₂]^2-. At higher pressures of N₂, all the octahedral holes in the structure become occupied by [N₂]^2- ions, and the final product, SrN, is better formulated as (Sr₂‏)₄(N^3-)₂(N₂^2-).

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