Oxidation of Coordinated Ligands

In the foregoing section coordinated dioxygen was reacted with various substrates resulting in either oxidation and incorporation into the coordination sphere or oxidation and expulsion from the coordination sphere. Clearly the latter process is the most desirable one to extend to catalytic reactions. In this section reactions of coordinated ligands with molecular oxygen are considered. In many cases, the results are the same as those reviewed in the previous section suggesting the possibility of common intermediates. This is particularly true in the reactions of SO2 complexes, equations (41)-(46). Literature references again appear under the arrow for these reactions. 

It should be pointed out, however, that although many of the dioxygen complexes react readily with SO2 to give sulfato complexes, many of the analogous SO2 complexes react more slowly, or not at all with molecular oxygen. Reactions of coordinated CO also give rise to carbonate complexes except in the case of rhodium complexes which tend to form coordinated or free CO2, equations (47)-(50). 

Fewer examples of the oxidation of coordinated NO exist, however, some ruthenium complexes are among those which undergo this reaction, equation (51). 

Inspection of equations (41)-(51) reveals some interesting correlations concerning the relative reactivity of several ligands in the coordination sphere of various metals. For instance, one might conclude from equations (43), (49) and (51) that the order of reactivity of ligands toward O2 in ruthenium complexes is SO2 NO, CO CNR, Ph3P. Coordinated phosphines, however, do react with oxygen under the proper conditions and, in fact several ruthenium complexes are catalysts for phosphine oxidation. In most instances when coordinated phosphines are oxidized they are readily expelled from the coordination sphere of the metal, however, in the case of Co(Il), the phosphine oxide is retained, equation (52). 

Since metal hydrides and metal alkyls are often intermediates of catalytic reactions of unsaturated hydrocarbons, reactions of these species with O2 are of interest. Cationic hydrido complexes of Ir, Rh, Ru and Os react with molecular oxygen to insert oxygen between the metal and the hydrido ligand, equations (53)-(56). 

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Oxidation of coordinated ligands in complexes [Fe(diimine)3] has been reviewed. " ... 

In addition to oxidation of metals or metal ions in coordination compounds, the NO " ion is used for oxidation of coordinated ligands. The ability of nitrosonium salts to oxidize coordinated azide ion has been applied extensively in preparative coordination chemistry since the reaction was discovered in 1968,34 pq. 8.11 ... 

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