Compounds with nitrogen

Key points: The cyanideion, CN, forms complexes with many d-metal ions; its coordination to the active sites of enzymes such as cytochrome c oxidase accounts for its high toxicity. Hydrogen cyanide, HCN, is produced in large amounts by the high-temperature catalytic partial oxidation of methane and ammonia, and is used as an intermediate in the synthesis of many common polymers, such as poly (methyl methacrylate) and poly(acrylonitrile). It is highly volatile (b.p. 26°C) and, like the CN ion, highly poisonous. In some respects the toxicity of the CN ion is similar to that of the isoelectronic CO molecule because both form complexes with iron porphyrin molecules. However, whereas CO attaches to the Fe in haemoglobin and causes oxygen starvation, CN targets the Fe in the active site of cytochrome c oxidase (the enzyme in mitochondria that reduces oxygen to water), which results in a rapid and catastrophic col lapse of energy production.

Unlike the neutral ligand CO, the negatively charged CN ion is a strong Brønsted base (pKa=9.4) and a much poorer Lewis acid π acceptor. The CO ligand can form complexes with met als in a zero-oxidation state as it can remove electron density through the π system. However, the coordination chemistry of CN is more often associated with metal ions in positive oxidation states, as with Fe₂ in the hexacyanoferrate (II) complex, [Fe(CN)₆]^4- , as there will be less electron density on the metal ion. The toxic, flammable gas cyanogen, (CN)₂ (18), is known as a pseudohalogen because of its similarity to a halogen. It dissociates to give .CN radicals and forms interpseudohalogen com pounds, such as FCN and ClCN. Similarly, CN is an example of a pseudohalide ion (Section 17.7).

The direct reaction of Si and N₂ at high temperatures pro duces silicon nitride, Si₃N₄. This substance is very hard and in-ert, and is used in high-temperature ceramic materials. Current industrial research projects focus on the use of suitable organo silicon nitrogen compounds that might undergo pyrolysis to yield silicon nitride fibres and other shapes. When SiO₂ is heated with carbon, CO is evolved and silicon carbide, SiC, forms. This very hard material is widely used as the abrasive carborundum.Trisilylamine, (H₃Si)₃ N, the silicon analogue of trimethylamine, has very low Lewis basicity. It has a planar structure, or is fluxional with a very low barrier to inversion. The low basicity and planar structure have traditionally been attributed to d-orbital participation in bonding, allowing sp² hybridization around the N atom and delocalization of the lone pair through π bonding. However, quantum mechanical calculations indicate that whereas d orbit als play a role in delocalization, they are not responsible for the planar structure. Because the electronegativity of Si is lower than that of C, the Si-N bond is more polar than the C-N bond. This difference leads to long-range electrostatic repulsion between the silyl groups in trisilylamine and hence a planar structure.

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مواضيع ذات صلة

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides

Nitrogen activation

Microporous solids

Aluminosilicates

Oxides of germanium, tin, and lead

Compounds of carbon with oxygen and sulfur

Tin and lead halides