Nitro-nitrito isomerism

Despite the fact that the Werner complexes have been intensively studied, there appears to still be controversy on the extent of reversibility of the nitro-nitrito isomerization of [Co(NH3)2(N02)] ( 120-124) a property of interest in these kinds of complexes (vide infia). Recently, Eslami showed by DSC experiments that the thermal isomerization of pure [(NH3)5Co... 

A number of reports - have appeared of studies of the rates of aquation of cw-[Co(en)2(amine)X] + ions and the available data are also collected in Table 4. When X=NOr and amine=MeNH2, BuNHz, PhCHjNHz, py, PhNHg, or HO(CH2)nNHa (n = 2 or 3) the rate constants are very similar at 343 K, all of the values falling in the range (1.01—1.40)x 10 s . A mechanism involving nitro-> nitrito isomerization in the rate-determining step is postulated in this case. ... 

Boldyreva EV (2001) Structural aspects of intramolecular solid-state linkage isomerization in Co(III) pentaammine nitro/nitrito complexes. Russ J Coord Chem 27 1-28... 

Werner knew of two other examples of linkage isomerism, both nitro-nitrito isomers, and they underwent the same period of skepticism and confirmation as the compounds discussed above although considerably less work was done with diem. A period of more than 50 years passed before Basolo and coworkers attacked the problem with rather amazing results.w Linkage isomerism, once relegated to a few lines as an "exceptional" situation in discussions of isomerism, now boasts an extensive chemistry which continues to develop. The first new linkage isomers prepared were mtro-nitnto isomers of Cr lII), Rh(lll), Ir(lll), and Pt(lV). In all cases except Cr(lll), the nitrito isomer converts readily to the more stable nitro isomer... 

In valence isomerization reactions the most common case is the rearrangement of a ligand, as in the example in Figure 4.79 in which a nitro group, N02, isomerizes to a nitrito group, O—N=0. The primary photochemical process is in fact a dissociation of the Co-N bond, with formation of free radicals. The nitro radical isomerized to its nitrito form, then binds again to make the new complex. 

A transient species has been detected in a 77 K, 365 nm solid-state photochemical study of the nitro nitrito linkage isomerization of [Co(NH3)5(N02)]Cla. This transient is postulated to involve a bidentate nitro-group in which a nitrogen and oxygen atom of the nitrite ion are co-ordinated to the cobalt(m). ... 

Fig. 3.18 Fully reversible nitro-nitrito linkage IsomerismLlnkage isomerism in a single crystal, in -N02-containing complex of Ni(II), [Ni(Et4dien)(i7 -0,0N)(ij -N02)]. Ni turquoise, N blue, O red) Et4dien = Af,Af,A, A -tetraethyldiethylenetriamine. Copyright John Wiley Sons 2011 [96]...

Hatcher LE, Warren MR, Allan DR et al (2011) Metastable linkage isomerism in [Ni(Et4dien)(N02)2] a combined thermal and photocrystallographic structural Investigation of a nitro/nitrito interconversion. Angew Chem Int Ed 50 8371-8374... 

The reader is referred to Section 5.7.6.2 for discussion of nitrito to nitro linkage isomerization kinetics at rhodium(III). ... 

The nitrito-nitro linkage isomerization of [Co(NH3)5(ONO)] in liquid ammonia has been shown to proceed entirely by an intramolecular conjugate-base mechanism. The use of liquid ammonia as solvent allowed the separate determination of the pre-equilibrium constant Kqb (AH = 27 kJ moP A5 = -14 J moP ) and the rate constant fccB for the ratedetermining step = 78 kJ moP = 29 J moP ). 

Other more stable salts of this type, such as the substances [Co en2(ONO)2]R and [Co en2(N02)2]R, contain the (ONO) and the (N02) group respectively. The former are brick-red in colour, the latter are brown. The isomerism is due to these groups which arc distinguished as nitrito, (ONO), and nitro, (N02).2> 3... 

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