Ligand effects and kinetics

we are facing an impossible task to summarize the ligand effect for monodentate phosphines (and phosphorus ligands) and to give credit to the numerous contributions. For the sake of didactics we will present a few rules of thumb, which at this point will not be fully exemplified by literature data, but which will be supported when diphosphines are discussed in section 8.3. 

More sterically demanding ligands will favour the formation of species containing a low number of ligands L and therefore more CO ligands. A high proportion of CO ligands also leads to electron poor rhodium species and thus to enhanced dissociation of CO. For phosphites this effect has been clearly observed [40] as will be discussed in section 8.4. 

The observed order in propene concentration is less than one, which might point to saturation kinetics. Indeed, high concentrations were used, but perhaps the non-ideal behaviour of propene (critical temperature 94 °C) plays a role in this. Under similar conditions for 1-hexene and 1-octene a neat first order behaviour in alkene has been observed using Rh-PPh3 catalysts [36,42], 

At high PPh3 concentrations, where the catalyst resting state is (PPh3)3Rh(CO)H, phosphine dissociation must occur to form the coordinatively unsaturated intermediates 3c and 3t. This dissociation is suppressed by increased PPh3 concentration, which serves to reduce the concentration of active Rh species in the catalytic cycle. 

At lower PPh3 concentrations where the predominant resting state observed by in situ studies is (PPh3)2Rh(CO)2H, species 3c and 3t are formed by CO dissociation, which is likewise inhibited by increased CO concentration. Consistent with this mechanism is the recent determination that dissociation/association of CO is reversible and faster than hydroformylation for arylphosphines (see 8.3). An inverse order in CO pressure and a zero order dependency on H2 pressure was reported by several authors [32,42,43], Under standard conditions, we propose that the best starting point for the kinetics is an equation of the type  

---