Abstract Mechanistic aspects of the hydrogenation of ketones employing cationic rhodium (I) complexes [Rh ((i-Pr) 2 P (CH 2) 4 P (i-Pr) 2)(NBD)] ClO 4 (NBD= norbornadiene) and [Rh (CyDIOP)(NBD)] ClO 4 (CyDIOP= 2, 3-O-isopropylidene-2, 3-dihydroxy-1, 4-bis ( dicyclohexylphosphino) butane) and a neutral complex,“Rh-(CyDIOP) Cl” were studied. The cationic complex-catalyzed hydrogenation of the poor coordinating simple ketone ...