Reactions of Cobalt m

Complex 165 reacts with CO at room temperature to give />-tolyl isocyanate and the golden dicarbonyl compound 166. The latter can also be generated by reaction of 164 with excess of CO and converted back to imide 165 by the addition of 2 equiv of /> toly 1 azide (2003JA322). Reaction of cobalt(m) imide 167 analogous to 165, available from [PhB(CH2PPh2)3]CoI 156 by the same procedure as for 165, with CO proceeds similarly <2002JA11238>. 

Whilst the majority of the available evidence may be interpreted in terms of a dissociative type of mechanism for substitution, there is a substantial number of reactions for which the kinetic data indicate a purely associative SN2 mechanism. In particular, it was found that reactions of cobalt(m) ammines in basic conditions obeyed pure second-order kinetics (Fig. 5-39). 

Figure 5-40. The first step in the reaction of cobalt(m) ammine complexes with hydroxide often involves deprotonation of an ammine ligand to give an amido species.

Figure 7-15. Some reactions of cobalt(m) diamsar. The nitro groups of the ligand may be reduced to amino groups, which may then be diazotised and converted to halogen substituents. Throughout these conversions, the cobalt ion remains within the encapsulating ligand.

The preparation of many metal complexes often involves an accompanying oxidation-reduction reaction. For the thousands of cobalt(III) complexes that have been prepared, the starting material was almost always some cobalt(II) salt. This is because the usual oxidation state of cobalt in its simple salts is 2. The oxidation state of 3 becomes the stable form only when cobalt is coordinated to certain types of ligands (Section 5.2). Furthermore, it is convenient to start with salts of cobalt(n) because cobalt(ii) complexes undergo substitution reactions very rapidly, whereas reactions of cobalt(m) complexes are very slow (Section 6.4). The preparation of cobalt(ni) complexes, therefore, proceeds by a fast reaction between cobalt(ll) and the ligand to form a cobalt(ll) complex which is then oxidized to the corresponding cobalt(ni) complex. For example, reaction (19) is presumed to involve first the formation of [Co(NH3)6] ... 

The radical cation intermediate formed in the primary redox reaction has been characterized using e.s.r. techniques. The reaction of cobalt(m) acetate with p-toluic acid is autocatalytic in acetic acid, the second-order dependence on oxidant concentration being seen to reflect the dimeric nature of the metal-ion reactant. The first step involves a redox equilibrium with one mole of Co released, which thus explains the observed inverse order of [Co ] on the rate. The radical intermediate in this system may then react with any Og present. 

In recent years interest in the substitution reactions of cobalt(m) complexes has... 

Further data have been reported on the reactions of cobalt(m) porphyrins with diazoalkanes as a route to vinyl- or halomethyl-cobalt(m) porphyrin complexes. The reaction pathway, involving probable insertion of an initially generated carbene fragment into the Co—N bond is also discussed. ... 

The effects of H+, Co , F , and Ag+ on the reaction of cobalt(m) with water have been examined, and in the oxidation of diacetone alcohol there is indirect evidence for an intermediate complex from Michaelis-Menten plots. 

Several examples are known of the effectiveness of organic compounds sensitizing photochemical reactions of cobalt(m) and of chromium(m) complexes, and biacetyl is known to photosensitize the aquation of [PtCU] ". Now the first example of photosensitization of substitution at platinum(n) by an inorganic species has been reported [Ru(bipy)3] + sensitizes photoaquation of the [PtCl4] anion. ... 

The reactions of cobalt(m) and ruthenium(m) ammine complexes with Yb , Eu, and Sm have been investigated using a pulse radiolysis technique. Whereas the reaction rates for Eu are similar to those of previous workers, the data for Yb + are in marked contrast to those described above. A previous... 

Kinetics and mechanisms of substitution reactions of cobalt(III) frans-dioximes, in non-aqueous media. N. M. Samus and A. V. Ablov, Coord. Chem. Rev., 1979, 28,177-203 (47). 

The complexation of coordination compounds may make it possible to control their photochemical behaviour via the structure of the supramolecular species formed. For instance, the binding of cobalt(m) hexacyanide by macrocyclic polyammonium receptors markedly affects their photoaquation quantum yield in a structure-dependent manner [8.73-8.77]. It thus appears possible to orient the photosubstitution reactions of transition-metal complexes by using appropriate receptor molecules. Such effects may be general, applying to complex cations as well as to complex anions [2.114]. 

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... 

The electrophilic reactions of co-ordinated 1,10-phenanthrolines are not always as simple as might be expected. Thus, the nitration of cobalt(m) 1,10-phenanthroline complexes yields 5-nitro-1,10-phenanthroline derivatives at low temperature, but prolonged reaction in hot solution leads to further reaction and oxidation of the ligand to give excellent yields of 1,10-phenanthroline-5,6-quinone complexes (Fig. 8-40). Even after the formation of the quinone, the complexes may exhibit further reaction. For example, reaction of the l,10-phenanthroline-5,6-quinone complexes with base results in the formation of a complex of 2,2 -bipyridine-3,3 -dicarboxylic acid (Fig. 8-41)... 

A study of the kinetics and mechanism of the oxidation of m-anium(m) by cobalt(iii) in perchlorate media showed the rate to be independent of acid concentration. In common with many other reactions of cobalt(iii) the rate constant was found to be lower than expected from a consideration of the very high AG° value. The kinetics and mechanism of the oxidation of uranium(iv) by hypochlorous acid in aqueous acidic perchlorate media... 

Both Co2(CO)g and Co4(CO),2 are oxidized by air even in the solid state, and by halogens. With the latter, complete decomposition to CO and Co(II) halides occurs. Reaction of cobalt carbonyl derivatives with I2 in pyridine is frequently used for analyses by gas volumetry of CO evolved. Octacarbonyldicobalt, Co2(CO)g, is attacked by organic substances that contain active protons and converted to the corresponding Co(II) salts , as in reactions for Fe(CO)j [equations (1) and (m), 14.6.2.3.2]. Included here is reaction of Co2(CO)g with cyclopentadiene to give dicarbonylcyclopentadienylcobalt(I) as a dark red oil (b.p. 75 C/22 torr) ... 

Conlon M, Johnson AW, Overend WR, Rajapaksa D, Elson CM (1973) Structure and reactions of cobalt corroles. J Chem Soc Perkin Trans 2281-2288... 

The reaction of cobalt(II) tetrasulfophthalocyanine, [Co(II)(tspc)] , with CH3, CH2CH2OH, CH(CH3)CH20H, CH(CH3)CH(CH3)0H, and CH2C(CH3)20H free radicals has been studied. Results indicate the initial formation of a [Co(II)(tspc-R)] species where the exact nature of the interaction of R and tspc is not clear. There follows a subsequent internal redox formation of [(tspc)Co(III)-R] via a first-order process. Subsequent decomposition produces methane for R or alkenes for ROH. Interaction of methyl radical with [Co(II)(nta)(H20)2]" [nta = N(CH2C02)r] yields [(nta)(H20)Co(III)-CH3] (Ki = 2.7 0.5 X 10 M and k i = 60 10 This reaction is followed by... 

The fluorination of pentafluoro-, 3-chlorotetrafluoro-, and 3,5-dichlorotrifluoro-pyridines with a mixture of cobalt(m) and calcium fluorides gives mixtures of perhalogenoazacyclohex-enes and -adienes and perhalogeno-l-azahexenes, depending on the reaction conditions used (see p. 219) tetrafluoropyrazine gives 40% of perfluoro-l,4-diazacyclohexa-l,3-diene (288), whereas tetrafluoropyrimidine affords the dimer (289), which is evidence for the intermediacy of radicals. ... 

Table 1 Activation volumes (AF ) and volumes of reaction (AF) for aquation of cobalt m) and chromium(jii) complexes...

Another pieee of evidenee to support the 7d CB mechanism is that if there is no N-H hydrogen present in a cobalt(m) complex, the complex reacts slowly with OIT. This suggests that acid-base properties of the complex are more important to the rate of reaction than are the nucleophilic properties of OIT. This base hydrolysis reaction of cobalt(ni) ammine complexes illustrates the fact that kinetic data often can be interpreted in more than one way and that rather subtle experiments must be performed to eliminate one or more possible meehanisms. 

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