Hydrogenation catalyst species, PPhs

Rhodium. In addition to the studies on the alkene hydrogenation catalyst [RhCl(PPhs)3] (see Vol. 3), some other very detailed studies on this system have appeared. It is now apparent that the complexes [RhClLJ [L = PPh, or P(p-tolyl)3] do not dissociate to a spectroscopically detectable extent. However, despite its low concentration a species of composition [RhClLJ must be postulated, as previously reported, in order to explain the kinetics of H2 reactions with solutions of [RhOLs]. The complexes [RhClLa], howev, are in equilibrium with the dimers, [RhClLJa. The dimer reacts with H2 to form (47) and is also readily cleaved with ethylene or L, but not cyclohexene, to form [RhCl(C2H4)L2] or [RhClLJ. ... 

RhCl(PPh- ) as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

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