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Date: 13-11-2018
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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-(N3-). 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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مخاطر خفية لمكون شائع في مشروبات الطاقة والمكملات الغذائية
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"آبل" تشغّل نظامها الجديد للذكاء الاصطناعي على أجهزتها
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تستخدم لأول مرة... مستشفى الإمام زين العابدين (ع) التابع للعتبة الحسينية يعتمد تقنيات حديثة في تثبيت الكسور المعقدة
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