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Contrary to earlier speculation, mononuclear rhodium species that contain trans diphosphines, such as SPANphos, are inactive towards the oxidative addition of methyl iodide, whereas the corresponding dinuclear species are very reactive. Thus, the active species in methanol carbonylation is most likely the dimeric system (see scheme; P orange, O red, Rh blue, Cl green).
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Freixa et al. (2005) studied this question.
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