Nitrito-to-nitro isomerization

The most important feature of yet another investigation of nitrito to nitro isomerization of complexes [M(NH3)5(ONO)] ", with M = Rh and Ir (vide infra) as well as Co, is the discovery of a hitherto undetected base catalysed pathway [equation (33)]. 

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

The spontaneous isomerization from the nitrito (—ONO) to the nitro (—N02) pentaammine-rhodium(III) complex has been well studied, and compared to the isomerizations of the analogous Co, Ir and Cr complexes. Isomerization was shown to occur in both the solid state and, about an order of magnitude more rapidly, in aqueous solution.517 The volume of activation (AF ) in aqueous solution is comparable for [M(NH3)5ONO]2+ (M = Co, Ir and Rh, AF = —6.7, —5.9 and — 7.4 + 4 cm3 mol-1 respectively), which is consistent with the anticipated intramolecular isomerization.584 The linkage isomerization is catalyzed by OH-,585 and by Hg2+, 5B6 In alkaline solution, the linkage isomerization of nitritopentaamminerhodium(III) obeys the rate expression rate =... 

The effect of placing thioether linkages trans to a site of nitrito substitution and its effect on the subsequent nitrito nitro isomerization has been studied using the [CoL(OH2)] cation where L = 1,11-diamino-3,6,9-trithiaundecane (10). 

A similar rate law operates for isomerization of the cis and trans forms of [Co(en)2(ONO)2] and of [Co(en)2(ONO)(N02)] these nitrito nitro iso-merizations take place without change in configuration at the cobalt. Similarly nitrito nitro isomerization of ( + )-c/5-[Co(en)2(ONO)(N02)] occurs without loss of chirality. These stereochemical results contrast with normally observed behavior in substitution at cobalt(III). The nitrito nitro isomerization of [Co(NH3)5(ONO)], in this case generated in situ photochemically, has also been studied in poly(vinyl alcohol) media. This very viscous medium, viz. pva sheets, was used to probe the gap between water and solid state studies. The rate of isomerization in pva is similar to that in water, but about ten times faster than in the solid phase. Such a short range of reactivities is, of course, consistent with an intramolecular mechanism in all three media. ... 

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... 

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. 

---