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. ... 

The tight 7t-bonded intermediate is consistent with a negative volume of activation. It also explains the (unanticipated) O-to-O exchange in the nitrito ligand at a rate comparable to the O-to-N isomerization (Sec. 2.2.2). ATj/Atj = 1.2 ... 

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... 

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... 

Ionisation Metamerism.—Numerous examples of this type of isomerism occur in the cobalt-ammine series, such as the two metameric substances, ehloro-nitrito-tetrammino-cobaltic chloride, [Co(NH3)4Cl. NOJC1, and dichloro-tetrammino-cobaltic nitrite, [Co(NH3)4C12]N02.1... 

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... 

The reaction of [Pt(NH3)5(H20)]Cl4 with NaN02 in hydrochloric acid at 0°C yields the O-bonded nitrito complex [Pt(NH3)5(ONO)]Cl3, which isomerizes to the thermodynamically more stable N-bonded isomer (equation 479).1630... 

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. 

Look at the colors of the isomeric complexes in Figure 20.17, and predict which is the stronger field ligand, nitro (-N02) or nitrito (-ONO) Explain your reasoning. 

Linkage isomerism in metal complexes was discovered in 1894 by Jorgensen.1 He prepared and characterized the nitrito... 

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