Nitrito-nitro linkage isomers

It is noteworthy that the successful synthesis of nitrito-nitro linkage isomers for other metal ammines was designed only after the mechanism of formation of [Co(NH3)50NO]2+ and its rearrangement had been elucidated.4 It was found that the nitrito complex is readily formed without the cleavage of the Co—0 bond ... 

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. 

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... 

Steric factors may play an important role in stabilizing one or the other of a pair of linkage isomers. Thus nitro-nitrito, thiocyanato-isothiocyanato, and selenocyanato-isoselenocyanato pairs differ in steric requirements (Fig. 12.45). 

Of particular significance in this reaction scheme is the 0-nitrosation step (18), which suggests that the Co-O bond is not cleaved and that the kinetic product is therefore the unstable nitrito (Co-ONO) isomer. This then rearranges to the stable nitro (C0-NO2) linkage isomer. Oxygen-18 labelling experiments of Mur-mann and Taube later confirmed that there is no rupture of the Co-O bond during this process. 

In a few cases ambidentate ligands give rise to linkage isomers, that is, complexes that differ only injhe mannerln which one or more-ambidentate— ligands are attached to the metal atom. The first example, discovered by S. M. Jorgensen in 1894, involved the nitro and nitrito isomers of [Co(NH3)5N02]C12. The nitro isomer here, and apparently in all other cases, is the stable one. With NOJ, however, many further complexities arise because of the several other (bidentate and bridging) modes of attachment that can occur (see Section 21-8). 

The red nitritopentammine complex, [Co(NH3)5(ONO)]Cl2, is unstable and is gradually converted to the stable yellow nitro complex. The kinetics of this conversion can be studied by observing the disappearance of the nitrito bands. Burmeister reviewed the vibrational spectra of these and other linkage isomers. 

Raithby and coworkers have observed the reversible 100% conversion of the red [(dppe)Ni( 7 -N02)Cl] complex to its photoinduced yellow linkage isomer [(dppe)Ni( 7 -ONO)Cl] (Equation 2) (135). Raman spectral data revealed a band at 1333 cm in the parent nitro compound and a 1404 cm band in the nitrito isomer. The nitrito isomer was further identified by photocrystallography and was found to be stable between 100 and 160 K and observed to reconvert to the ground state at 300 K. 

Certain ligands can bind to metal ions in more than one way. One example of such a ligand is nitro, — N02, and nitrito, —ONO .The donor atom is written first in the chemical formula. An example of linkage isomers is shown on page 1001. 

The nitrito kinetic product finally rearranges by an intramolecular process to the stable nitro (M—NO2) linkage isomer, Eq. (78). 

Referring to the mechanism proposed [Eq. (77)] for the formation of [Co(NHs)60NO]2+, it is of interest to note that, making use of this information, it was possible to prepare other nitrito complexes. Jorgensen had prepared the nitrito (M—ONO)-nitro (M—NO2) linkage isomers of Co (III) 70 years ago. Attempts to prepare such linkage isomers of other metals failed until after the elucidation of the mechanism of formation of... 

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