Cationic rhodium and iridium complexes of the type [M(COD)(PPh3)(2)]PF6 (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2-5 bar) and temperatures (60 'C for Rh and 100 'C for 1r) in toluene solution; the linear to branched ratio (l/b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0 and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For both complexes, the reaction proceeds according to the rate law r(i) = K(1)K(2)K(3)k(4) [M][olef][H-2] [Co]/([CO](2) + K-1[H-2][CO] + K1K2K3[olef][H-2]). Both complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO)(3) (PPh3)(2)]PF6, M = Rh, 2a; Ir, 2b, and with syn-gas to yield [MH2(CO)(2) (PPh3)(2)]PF6, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH2(CO)(PPh3)(2)]PF6 entering the corresponding catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin promoted by an entering CO molecule is the rate-determining step of the catalytic cycle. (c) 2007 Elsevier BN. All rights reserved.