Nitro and Nitrito Complexes

The normal vibrations of the unidentate N-bonded nitro complex may be approximated by those of a planar ZXY, molecule, as shown in Fig. 111-19. In addition to these modes, the NO2 twisting and skeletal modes of the whole complex may appear in the iow-frequency region. Table III-11 summarizes the observed frequencies and band assignments for typical nitro complexes. It is seen that these complexes exhibit I uCNOj) and /j(NOi) in the 1470-1370 and 1340-1320cm regions, respectively. On the other hand, the free NOJ ion exhibits these modes at 1250 and 1335 cm , respectively. Thus I uCNO ) 

If the NO2 group is bonded to a metal through one of its O atoms, it is called a nitrito complex. Table 111-12 lists the NO stretching frequencies of typical nitrito complexes. The two p(N02) of nitrito complexes are well separated, p(N = 0) and p(NO) being at 1485-140U and 1110-l050cm , 

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. 

The nitro group is known to form a bridge between two metal atoms. Nakamoto et al. suggested that among the three possible structures, IV, V, and VI. shown before. IV is most probable for 

Water in inorganic salts may be classifled as lattice or coordinated water. There is, however, no definite borderline between the two. The former term denotes water molecules trapped in the crystalline lattice, either by weak hydrogen bonds to the anion or by weak ionic bonds to the metal, or by both  

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Detailed discussion of the properties of Mn(C0)5N02 is outside the scope of this paper. It is of interest, however, that when heated at 65° under a vacurnn, the compound sublimes, and the product appears from its infrared spectrum to be a mixture of nitro and nitrito complexes. Some conversion of the Mn-N02 bond to Mn-ONO therefore occurs during the process of vaporization and condensation. 

Table 15 Structures Some Recent Nitro and Nitrito Cobalt(III) Complexes...

A few ligands are ambidentate, meaning that they can coordinate through alternative ligand atoms. Examples are nitrite (which can bind through N or O) and thiocyanate SCN- (S or N). The nomenclature N-nitrito and O-nitrito is recommended for complexes where in formulae the ligand atom is underlined, M-EsIO, ancj 1V1-ONO, respectively (although the nonsystematic names nitro and nitrito are also used for complexes). 

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). 

Unlike Co—NO complexes (Section 47.4.1.3), the reactivity of coordinated nitro and nitrito ligands toward electrophiles is not well explored. However Tovrog and his colleagues at Allied Chemical Corporation have recently shown that (28) catalytically oxidizes PPhj in a bimolecular transfer of O from coordinated NO2 (equation 23), ° and that (28) and the similar porphyrin complex [Co(TPP)(py)(N02)] both catalyze the oxidation of primary and secondary alcohols to aldehydes and ketones in the presence of the Lewis acids BFj HjO or LiPF non-radical pathways are proposed. Internal redox in [Co(N02)2(bipy)2]X also occurs on heating, with oxygen transfer, to give the Co complex [Co(N03)X(bipy)2] and NO (X = Cl , Br , I , CIO3 , CIO ). ... 

Table 111-13. Vibrational Frequencies of Ni ll) Complexes Containing Nitro and Nitrito Groups (cm" )...

The Cr-N02 complexes reported to date do not appear to isomerize under photolysis conditions, but can decompose with release of NO when irradiated with light (88). However, the complex CpCr(N0)2(N02) (Cp = cyclopentadiene) prepared by metathesis of CpCr(NO)2Cl with NaN02 in methanol, exists in both the nitro and nitrito forms in solution (Figure 8), with the nitrito isomer favored both in solution and in the solid state (89). The equilibrium is temperature dependent as determined by H NMR spectroscopy, with the nitrito isomer favored at higher temperatures (AHeq = 1.8(3)kcalmol AS=9.3(9) eu) (89). In addition, H NMR and... 

T FIGURE 20.17 Samples and structures of (a) the nitro complex [Co(NH3)5(N02)]2+, which contains an N-bonded N02- ligand, and (b) the nitrito complex [Co(NH3)5(ONO)]2+, which contains an O-bonded N02-ligand. 

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 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 =... 

This mechanistic concept for the formation of nitrito-complexes of cobalt(iii) suggests that other analogous metal systems should yield similar materials. However, the corresponding nitrito-complexes of rhodium(iii) and of iridium(iii) were not known. One reason that previous investigators had not been successful in preparing these is that the platinum group metal complexes are usually very slow to react and rather drastic reaction conditions had been used. As a result, the stable nitro-product rather than the kinetic nitrito-product was isolated. Since the formation of M-ONO does not involve M-O bond cleavage, the reaction as shown in (18) is expected to occur even under rather mild experimental conditions. This was found to be the case and salts of the new complexes [M(ONO)(NH3)5] where M = rhodium(iii), iridium(iii) or plati-num(iv) have been prepared. ... 

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