Tungsten complexes oxygen atom transfer

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

An additional aspect in studying the half-reactions arises when a reaction is not treated as pseudo first order reaction. This is the case when the substrate is in approximately the same concentration as the metal complex. Caradonna et al. for instance studied the required treatment for such a case. Due to the accumulation of the product in the course of the reaction, the back reaction is not negligible anymore. To the best of our knowledge, analyses of the back reaction have never been mentioned in the literature with respect to oxygen atom transfer reactions of model complexes and this is due to several reasons. Either the reaction rate is too small compared to the forward reaction so that experimental difficulties in determining the reaction rate constant emerged. Or, as was emphasized by Enemark et No bis(dithi-olene) molybdenum or tungsten eomplex was observed to reduce PhjPO . ... 

The heterometallic oxo-acetylide complex [Cp Re(0)(p-PhCCPh)W(C0)2Cp] (54), possessing an 0x0 ligand bound to the rhenium atom, was prepared S by treatment of [ Cp Re(0)(p-0))2] with the acetylide complex [CpW(CO)3(CCPh)]. Thermolysis of the product afforded the isomeric complex [Cp Re(C0)2(p-PhCCPh)W(0)Cp] (55), showing an internal transfer of oxygen atom from rhenium to tungsten atoms. 

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