Cobalt and Iridium

This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

DR. NORTON I read that paper with some care. I am afraid I am not convinced by the result. The cobalt and iridium results are believable. They have been carefully measured and they agree with the general trend. The rhodium result is not well measured, and I am inclined, therefore, to be suspicious of the fact that it doesn t fit the general trend. 

According to the Periodic Classification, rhodium, which in its properties forms a mean between cobalt and iridium, and betw een ruthenium and palladium, should have an atomic weight intermediate between the values for these extreme elements, namely, from 102 to 105. 

Rhodium sulphate, like its analogues the sulphates of cobalt and iridium, yields stable salts with sulphates of the alkali metals known as alums. These are well-defined crystalline salts, isomorphous with the better known iron and aluminium alums. They thus form an interesting link between these metals and the central vertical column in Group VIII, of which rhodium is the middle member. These alums... 

Comparison of Rhodium with Cobalt and Iridium Catalysts... 

Cobalt and iridium complexes can also function as hydroformylation catalysts, but of course the cobalt catalysts are much better known.t The reduced activity of Co and Ir compared to Rh has been attributed to the greater stability of their 18-electron complexes to ligand dissociatioii. The Co and Ir systems do provide some interesting comparisons with Rh, both in the behavior of the catalytic systems and in the identification of catalytic intermediates. 

For the cobalt- and iridium-catalyzed processes the intermediate catalyst complexes are shown in Figures 6.15.5 and 6.15.6, respectively. 

The dimeric complexes [M2(CO)io] (M = Mn or Re) continue to be actively studied. The presence of 17-electron free radicals, [M(CO)s], as intermediates in the thermal substitution reactions, and the chemistry of these and other 17-electron complexes are the questions being addressed. Substitution reactions of cobalt and iridium carbonyl clusters, with an attempt to define and separate electronic and steric effects, has also been an especially active area. This chemistry is discussed in Section 10.1.4. 

Attempts have been made to separate steric and electronic factors governing ligand substitution in tetranuclear clusters of cobalt and iridium. For a series of isostructural complexes [Co4(CO)nL], the rate of dissociative loss of CO has been determined by measuring CO exchange rates. The ligand L is a phosphine or phosphite, and occupies an axial position as shown in structure 21. The relative rates of CO dissociation in heptane... 

The remarks concerning the structure of organogermanium derivatives of metals stated in the preceeding chapter also stand for the cobalt and iridium compounds. The organogermanes listed in Table 86 are prepared by the following methods. 

Additional organogermanes containing cobalt and iridium listed in Table 86. 

Table 86. Bimetallic Organogermanium Derivatives of Cobalt and Iridium...

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