Clusters disproportionation

Another possible mechanism also involves a methyl radical (mechanism 3). In this case, electron transfer from one of the reduced clusters on CODH/ACS to methyl-Co(III) would form a methyl-Co(II) species that can disproportionate to form Co(I) and methyl-Ni(II). 

For the most electronegative ligand, fluorine, we expect a relativistic destabilization in the Au—F bond, which was indeed determined to be —0.36eV at the coupled cluster level [182,183], Nevertheless, AuF has a sufficiently high dissociation energy of about 3.17 eV and has been identified recently in the gas phase [184]. In solution or in the solid state it would disproportionate to metallic Au and compounds of Au (AuF3 for the solid). However, a carbene-stabilized Au(I) fluoride was synthesized only very recently (see discussion in the next section) [185]. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

Thus partial evaporation of HC1 during the synthesis of [Tc2C18]3 by the method of Eakins et al. [24] and by the autoclave method [22,42,43] creates favorable conditions (from the standpoint of the acidity of the solution and the concentration of technetium ions in it) for the formation of the octachloro-ditechnetates (+2.5), since, on the one hand, the rate of reduction of the technetium(IV) ions increases owing to their hydrolysis [42] and, on the other hand, the stability of the hydrolyzed cluster ions formed increases in relation to the reactions involving disproportionation and oxidation by atmospheric oxygen [9,52,80,87]. We may note that under the conditions of more pronounced hydrolysis, the rate of reduction of technetium(IV) increases so much that the formation of metallic technetium becomes possible. 

Such a classification of technetium cluster compounds, in our opinion, reflects the relationship between the thermal stability and structure of the clusters quite well. Moreover, on the basis of this classification it is easier to follow the mechanism of the main thermochemical transitions of technetium clusters, such as (1) dehydration (2) disproportionation and related processes occurring without changes or with only small changes in mass (3) one-stage processes of thermolysis. We shall now consider these main mechanisms of the thermochemical reactions of technetium clusters in greater detail. 

Clusters with K+ cations decompose according to the mechanism of the disproportionation reaction (10) [11,121],... 

Binuclear halogenide clusters with ammonium cations under quasi-isother-mal conditions also decompose by the disproportionation mechanism (33) [51,123]. 

The thermal decomposition of binuclear technetium sulfate clusters also occurs according to the disproportionation mechanism, but in this case, (a) other technetium-containing products are formed, and (b) a weight loss due to the evolution of gaseous products is also observed (36) [59]. 

Scheme 1. Metalloid MxXy clusters (x > y) as intermediates during the disproportionation reaction of MX solutions (M = Al, Ga X = Cl, Br, I).

The last step, the formation of metal, takes place if in the reducing atmosphere (MX excess) a critical cluster size is reached (e.g., by heating the solution to more than 100°C), thus forming a giant cluster whose metal core is nearly identical to the bulk material and which can disproportionate to metal and Mm species without any further significant change of its bondings. 

Support for these proposals comes from several sources first, the disproportionation reactions of mixed metal clusters such as Rh2Co2(CO)j2 or Fe2Ru(CO)12 ... 

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