Ternary boron nitrides

    Ternary boron nitrides (i.e. compounds of type MxByNz) are a relatively new addition to boron–nitrogen chemistry. The high-temperature reactions of hexagonal BN with Li₃N or Mg₃N₂ lead to Li₃BN₂ and Mg₃BN₃ respectively. Reaction 12.56 is used to prepare Na₃BN₂ because of the difficulty in accessing Na₃N as a starting material.

    Structural determinations for Li₃BN₂, Na₃BN₂ and Mg₃BN₃confirm the presence of discrete [BN₂]^3-ions, and Mg₃BN₃ is therefore better formulated as (Mg²⁺‏)₃[BN₂]^3-(N^3-). The [BN₂]^3- ion (12.23) is isoelectronic and isostructural with CO₂.

  Ternary boron nitrides containing d-block metal ions are not well represented. In contrast, lanthanoid metal compounds are well established, and include Eu₃(BN₂)₂, La₃ [B₃N₆], La₅[B₃N₆][BN₃] and Ce₃[B₂N₄] which are formulated as involving [BN₂]^3-, [BN₃]^6-, [B₂ N₄]^8- and [B₃N₆]^9- ions.

   These nitridoborate compounds may be formed by heating (>1670 K) mixtures of powdered lanthanoid metal, metal nitride and hexagonal-BN, or by metathesis reactions between Li₃BN₂ and LaCl₃. The ions [BN₃]^6-and [B₂N₄]^8- are isoelectronic analogues of [CO₃]^2_ and [C₂O₄]^2_, respectively. The B^_N bonds in [BN₃]^6- are equivalent and diagram in below shows a set of resonance structures consistent with this observation. The bonding can also be described in terms of a delocalized bonding model involving π-interactions between N 2p and B 2p orbitals. Similarly, sets of resonance structures or delocalized bonding models are needed to describe the bonding in [B₂N₄]^8-  and [B₃N₆]^9-.

   The solid state structures of La₃[B₃N₆], La₅[B₃N₆][BN₃] and La₆[B₃N₆][BN₃]N show that the [B₃N₆]^9- ion contains a six-membered B₃N₃ ring with a chair conformation (see below, B atoms are shown in orange). Each boron atom is in a planar environment, allowing it to participate in π-bonding to nitrogen.

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