Catalytic Cycle and Mechanistic Highlights

For PPhs as the ligand, a common starting complex is RhH(PPh3)3CO, complex 3, which under 1 bar of carbon monoxide forms complexes 2a and 2b, containing two phosphine ligands in equatorial positions or one in an apical position and the other in an equatorial position. Dissociation of either equatorial L or equatorial CO from 3 or 2 leads to the square-planar intermediates 4c and 4t (never observed) that have phosphines in cis- or trans configurations, respectively. Complexes 4 associate with 

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