Acids and oxy-ions

The existence of three oxy-acids of nitrogen is well established, nitrous, HNO2, nitric, HNO3, and hyponitroas, H2N2O2. The preparation of other acids or their salts has been described, but we shall comment on these compounds only if structural studies have been made. 

Nitrous acid. Only dilute solutions of nitrous acid have been prepared, and these rapidly decompose, evolving oxides of nitrogen. Two structures can be envisaged for the molecule of nitrous acid, the second of which could exist in cis and trans forms  

This tautomerism is suggested by the fact that by the interaction of metal nitrites with alkyl halides two series of compounds are obtained, the nitrites-in which the NO2 group is attached to carbon through an O atom—and the nitro compounds, in which there is a direct N—C link. From i.r. studies of nitrous acid gas it is concluded that there is no evidence for the existence of form (a) and that the two isomers of (b) exist in comparable amounts, the trans isomer being rather more stable. The structure (c) has been assigned to the trans isomer. 

Metal nitrites and nitrito compounds. Until recently the only stable simple metal nitrites known were those of the alkali and alkaline-earth metals, Zn, Cd, Ag(i), and Hg(i). The alkali nitrites decompose at red heat, the others at progressively lower temperatures. The ionic nitrites of the more electropositive metals contain the NOJ ion which in NaN02 has the following configuration ONO, 1154°, and N-0, 1-236 A. It may be of interest to summarize here the structural data for the unique series NOj, NO2, and NO2 with respectively 16,17, and 18 valence electrons  

The insoluble, more deeply coloured, nitrites of metals such as Hg are probably essentially covalent compounds. Anhydrous nitrites of Ni(ii) and Co(ii) have been made from NO2 and a suitable compound of the metal, for example, Ni(N02)2 from Ni(CO)4 and NO2 (each diluted with argon). It is stable up to 260°C. Complex nitrites of transition metals (e.g. Na3[Co(N02)6] and K2Pb[M(N02)6] (M = Fe, Co, Ni, Cu)) are well known. 

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