Nitrito

A typical value for the N O distance in nitro complexes is 124 pm whereas in nitrito complexes the terminal N-O (121pm) is shorter than the internal N-O(M) 129 pm. In the bidentate chelating mode (iii) the 2 M O distances may be fairly similar as in [Cu(bipy)2(02N)]N03 or quite different as in [Cu(bipy)(02N)2] ... 

It should be noted that, by convention, the ambidentate ligand is always written with its donor atom first, i.e. NO2 for the nitro, ONO for the nitrito, NCS for the A-thiocyanato and SCN for the 5 -thiocyanato complex. Differences in infrared spectra arising from the differences in bonding are often used to distinguish between such isomers. 

The hydroxypentammine is a useful starting material for the nitro and nitrito linkage isomers, the nitrito form separating under mild conditions but transforming to the nitro isomer on standing, especially when heated. 

Nitrito- Compounds. Organic compds containing one or several —O.N 0 groups which are called oxynitroso radicals. These compds are also known as organic nitrites. Some are expl, as, for example, Methyl Nitrite, CHj.O.NO (see in this Vol). They are not as stable as the corresponding nitrates and nitrocompds Ref Hackh s (1972), 457... 

A careful investigation of this feature suggests that it is attributable to N02 associated with both Bronsted acid and M+ cations [29]. The doublet at 1400 cm is identical in position and appearance to that observed in a sample of Na-Y doped with NaNOs [30]. We therefore assign this peak to a NO3- species affiliated with the residual sodium in the catalyst. The position of the band at 1528 cm- is very similar to that for nitrito species in Co-A and Co-Y [22, 23] and is, therefore, assigned to Co-ONO. The features at 1599, and 1574 cm- are best assigned to C0-O2NO [30]. The band at 1633 cm- is similar to that observed on H-, Na-, and Cu-ZSM-5. We believe that this feature is best assigned to nitrito (NO2) or nitrate (NO3-) species. 

A series of spectra taken during the TPD of NO into a stream containing 2.14% CH4 in He are shown in Figure 3. At temperatures up to 300 °C these spectra are identical to those presented in Figure 2. However, at 300 °C the nitrito peak at 1515 cm- [shifted from its position at 1528 cm-1 at room temperature] is notably absent, presumably due to the reaction of Co-ONO with CH4. 

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

Monodentate nitrito Bridging monodentate nitrito Monodentate nitrato Bridging monodentate nitrato... 

Chelating bidentate nitrito Bridging bidentate nitrito Chelating bidentate nitrato Bridging bidentate nitrato... 

Chelating nitro-nitrito Bridging nitro-nitrito ... 

Evaporation by heating a filtrate from precipitation of potassium cobaltinitrite caused it to turn purple and explode violently [1]. This was attributed to interaction of nitrite, nitrate, acetic acid and residual cobalt with formation of fulminic or methylnitrolic acids or their cobalt salts, all of which are explosive [2], Mixtures containing nitrates, nitrites and organic materials are potentially dangerous, especially in presence of acidic materials and heavy metals. A later publication confirms the suggestion of formation of nitro- or nitrito-cobaltate(III) [3],... 

One example was reported by Tolman and coworkers (78) who found that the copper(I) complex C Tp112 (TpR2=tris(3-(R2)-5-methylpyrazol-l-yl)hydroborate) promotes NO disproportionation via a weakly bound CuITpR2(NO) intermediate (formally a MNO 11 species). The products are N20 and a copper(II) nitrito complex (Eq. (36)). The rate law established the reaction to be first-order in copper complex concentration and second-order in [NO], and this was interpreted in terms of establishment of a pre-equilibrium between NO and the Cu(I) precursor and the Cux(NO) adduct, followed by rate-limiting electrophilic attack of a second NO molecule (mechanism B of Scheme 5) (78b). 

NO disproportionation has been shown to be promoted by the Mn(II) tropocoronand complex Mn(TC-5,5) (82) (Eq. (38)), and the reaction was found to involve three equivalents of NO leading to formation of N20 and O-coordinated nitrito ligand. The electron balance is provided by oxidation of Mn(II) to Mn(III). The mononitrosyl complex Mn(TC-5,5)(NO) was proposed to react with NO to produce an unstable cis-dini-trosyl, Mn(TC-5,5)(NO)2, which is then poised to form an N-coordinated hyponitrito (0=N-N=0) ligand from which oxygen transfer occurs to another NO (82a). The intermediacy of a hyponitrito ligand parallels other proposed mechanisms for metal complex promoted NO disproportionation (5a-d). 

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