Osmium catalysis addition with

The iodide reaction was first studied by incremental addition of thiosulphate ( The method of constant rates ) . This use of the method was attacked by Bell . Bray s work established the two rate terms shown in Table 28. The reaction has also been studied in the presence of arsenite , which does not interfere as thiosulphate does, and kj =3.3x10 l .mole .sec at 25 °C. Catalysis of the reaction has been demonstrated in presence of vanadium(IV) , ruthenium(III) , rhenium(II) osmium tetroxide " and iron(III) , together with retardation by Mn(II) - Ni(II) and chloride . These measurements were made at moderate acidities and appear to involve quite different dependence of rate on acid concentration from the reaction in absence of catalyst (half-order for Ru(OH), first-order for Re ). The mechanisms may also be quite different. 

Oxidation of phenol by tris(l,10-phenanthroline)osmium(III) is second order in Os(III) and phenol and inverse second order in Os(II) and acidity. A mechanism is inferred in which the phenoxyl radical is produced through a rapid proton-coupled electron transfer (PCET) pre-equilibrium, followed by rate-limiting phenoxyl radical coupling. Application of Marcus theory indicated that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it did not rule out the intervention of other pathways such as concerted proton-electron transfer or general-base catalysis DFT studies, at B3LYP/LACVP level, of the oxidation of ethylene by osmium tetroxide, osmyl hydroxide, and osmyl chloride indicated that in the reaction of osmium tetroxide, the [3 4- 2] addition pathway leading to a five-membered metallacycle intermediate is more favourable than the [24-2] addition. The reaction with osmyl hydroxide is less favourable. In the reaction with osmyl chloride, the [24-2] addition pathway is more favourable than the [3 4-2] addition. ... 

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