Loss of PPh3 from HRh(CO)(PPh3)2 generates considerably more active, but less regioselective hydroformylation
catalysts. The addition of excess phosphine ligand shifts the phosphine dissociation equilibrium back towards the more selective HRh(CO)(PPh3)2 catalyst. This explains why higher CO partial pressures lower the product regioselectivity, in marked contrast to what is observed for HCo(CO)4-catalyzed hydroformylation.