Cobalt complexes nitrito

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). 

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. 

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

Oxygen tracer experiments 25) confirmed that the cobalt-oxygen bond remained intact throughout the reaction. The nitrito complexes of penta-amminerhodium(III), pentaammineiridium(III), and pentaammineplati-num(IV) have been obtained by the same method (5). In a similar way, adding sulfur dioxide or sodium sulfite to solutions of aquopentaammine-cobalt (III) ion at low temperatures and low pH yields the unstable pink oxygen-bonded sulfito complex rather than the stable yellow-brown sulfur-bonded isomer. 

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. ... 

Photochemistry of cis-[Co(en)2(N02)2] in neat acetonitrile differs from that in water. In aqueous solution excitation of the complex ion in the ligand-to-metal charge transfer band gives the cobalt(II) ion and the nitrito linkage isomer. In acetonitrile, an intermediate cobalt(II) complex coordinated to NO2 is produced. As previous studies have shown, the solvent employed has a marked effect on the reactivity pattern of excited states. The viscosity dependence of the photoaquation of [Co(CN)j " and a variety of Cr(III) complexes has been studied using glycerol-water mixtures. " Cage effects are not important in the photosubstitution of CN in the systems studied. 

This mechanism parallels that proposed earlier for the aquation of nitrito-cobalt(m), -rhodium(ra), and -iridium(ni) complexes in acid media. ... 

Ligand Replacement Unidentate by Unidentate.—dissociative mechanism is proposed for the replacement of nitrito- or of thiocyanato-ligands in trans-[Co(dmgH)2(N02)2] and in rm/w-[Co(dmgH)2(NCS)2] respectively by thiourea. Thiocyanate is replaced much more rapidly than nitrite from these cobalt(m) centres. Substitution at bisdimethylglyoximatocobalt(m) complexes can be catalysed by cobalt(n) complexes. This has been demonstrated by the elucidation of the rate law for the cobalt(n)-catalysed reaction of trans-[Co(dmgH)2(PPhs)(NOa)] with pyridine ... 

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