Osmium complexes reduction rates

P(OMe)3] with Ati=4.2x 10 s" (30 °C). As with the osmium complexes just discussed this rate is several orders of magnitude lower than that of methane elimination from the corresponding methyl hydride while reductive elimination of methane from [Co(Me)2 P(OMe)s 4]+ exhibits an intermediate reactivity. The latter reaction is consequently thought to proceed via a multistage mechanism in which carbene complexes (resulting from a-hydrogen transfer to the metal) play an important role. Elimination of CH4 from the methyl hydride complex is probably a concerted process according to these workers. 

Tris(diimine)ruthenium(III) complexes are significantly more oxidizing than the analogous complexes of both iron(III) and osmium(III). This correlates well with the observation that rates of reduction in base are also faster for the tris(diimine)ruthenium(III) complexes. The tris(l,10-phenanthroline)ruthenium(III) reduction is significantly faster than the tris(2,2 -bipyridine)ruthenium(III) reduction, and this may be the reason why it is only the latter reaction that has been investigated in detail (1, 2). This system is particularly complex, and the rate law given by Eq. (1) holds only for very small concentrations of ruthenium complex. In contrast to the irondll) systems, simple kinetics... 

The oxidation of benzoin with cerium(IV) in perchloric acid solution is proposed to involve an interaction between Ce4+(aq.) ions and the keto alcohol, resulting in the formation of free radicals. The final product is benzoic acid.66 The rate of oxidation of crotyl alcohol with cerium(IV) is independent of the concentration of Ce(IV). The reaction induced polymerization of acrylonitrile indicating the formation of free radicals. The kinetics and activation parameters for the reaction have been determined.67 For the Ir(III)-catalysed oxidation of methyl ketones68 and cyclic ketones69 with Ce(IV) perchlorate, successive formation of complex between the reductant and Ce(IV) and then with the catalyst has been proposed. Results showed that in acidic solutions, iridium(III) is a more efficient catalyst than osmium and ruthenium compounds. 

It is interesting to note that the OsHCl(CO)(PR3)2 complexes catalyze the hydrogenation of styrene to ethylbenzene at faster rates than those for the alkyne reduction, suggesting that the origin of the selectivity is not kinetic. Thus, the stable alkenyl complexes represent a thermodynamic sink that causes virtually all the osmium in solution to be present as Os (E)-CH=CHPh Cl(CO)(PR3)2. Therefore, the kinetically unfavorable pathway becomes the only one available in the presence of alkyne. This thermodynamic difference is illustrated qualitatively in Figure 29. 

In the catalytic reaction, however, the dependence on reductant concentration differs from that previously, with retardation of the rate after an optimum value. The evidence for an initial osmium(vra)-tin(n)-chloro-complex has been substantiated by spectrophotometric studies. 

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