Metal nitrito complex

It is possible that in the use of alkali metal nitrites for nitrosyl synthesis, the first formed nitrito complex undergoes rapid ligand reduction similar to that above, as in equations (15) and (16). 

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 ... 

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. ... 

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" ,... 

A classical example of photochemical synthesis of a metal ammine complex is the conversion of a stable nitro to unstable nitrito complex, reaction (16). 

Vibrational spectroscopy is a powerful tool of deducing the mode of nitrite coordination in metal complexes. The nitro complex usually exhibits the N—O stretching absorptions in the 1300-1340cm" (vj and 1360-1430 cm" (v ) regions, while the v(N—O) and v(N=<)) bands from the nitrito complex appear in the 1050-1110 cmand 1400-1485 cm" regions, respectively. In addition, nitrito complexes lack the wagging modes near 620 cm which appear in all nitrito complexes. 

The O-bonded nitrito complex is thermodynamically less stable at room temperature and usually prepared at lower temperatures. Upon heating both in solution and in the solid state, it readily isomerizes to the stable N-bonded nitro form by an intramolecular process this was clarified by the tracer experiments in solution, and supported by high-pressure studies. At present. X-ray structures of a great number of metal complexes containing the N-bonded or O-bonded nitrite ligand are known. ... 

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... 

NO disproportionation has been shown to be promoted by the Mn(II) tropocoronand complex Mn(TC-5,5) (82) (Eq. (38)), and the reaction was found to involve three equivalents of NO leading to formation of N20 and O-coordinated nitrito ligand. The electron balance is provided by oxidation of Mn(II) to Mn(III). The mononitrosyl complex Mn(TC-5,5)(NO) was proposed to react with NO to produce an unstable cis-dini-trosyl, Mn(TC-5,5)(NO)2, which is then poised to form an N-coordinated hyponitrito (0=N-N=0) ligand from which oxygen transfer occurs to another NO (82a). The intermediacy of a hyponitrito ligand parallels other proposed mechanisms for metal complex promoted NO disproportionation (5a-d). 

A few examples will illustrate the use of those terms. Chloro-pentammino-cobaitic chloride is written as [Co(NII3)5.Cl]Cl2, nitrito-aquo-tetrammino-eobaltic chloride as [Co(N I3) (NH3),(N02).n20 (. l2, and chloro-aquo-tetrammino-eobaltic chloride as [Co(NI13)4C1(1120)JC1 2. It will be observed that the co-ordination number in each of the complexes is six, and this is characteristic not only of cobalt-ammines, but also of many other complexes. Some metals, however, yield complexes with co-ordination number four. For example, tetrammino-cuprio sulphate, [Cu(NH3)1]S04, and tctrammino-platinous chloride, [Pt(NH3)4]Cl2, belong to this class. On the other hand, the co-ordinatioi i number of hexannnino-platinic chloride, [Pt(NIl3)0]Cl. is six, but the central metallic atom is tetravalent, hence there are four chlorine atoms in the outer dissociable zone. 

On the basis of these observations and conclusions it was suggested that the cobalt complexes described above might follow a related pathway. Here, however, it can be argued that the presence of base in, e.g. the reactions of Co(dmgH)2NO, leads initially to the quantitative formation of Co,(dmgH)2(NO)B. In this case attack of 02 leads first to the peroxy compounds and then, because of the absence of a suitable NO+ compound, rearrangement occurs to give the nitrito derivative. Clearly whether or not nitro or nitrato compounds are produced would depend on the concentration of base B. In some cases it is possible to obtain mixtures of such products. These oxidations are also observed for other metals, e.g. equations (37) and (38). 

Figure 5-73. Hydroxide complexes of non-labile d6 metal centres undergo reactions with a selection of electrophiles. Of particular note is the specific method for the formation of monodentate 0-bon-ded complexes with nitrito, carboxylato, carbonato and sulfito ligands.

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