Complex [Rh(kappa(2)-P,P-dppe)(2)]acac ([1a]acac, dppe: 1,2-bis(diphenylphosphino)ethane) showed to be a highly efficient precatalyst for the hydroformylation of 1-hexene with formaldehyde under relatively mild reaction conditions (353-403 K) in 1,4-dioxane as the solvent; the corresponding C-7 aldehydes were obtained in linear to branched ratios (l/b) close to 2. A kinetic study for this reaction lead to the experimentally determined rate law: r(i) = {K(1)k(2)/(1 + K-1[CH2O])}[Rh][C6H12][CH2O] [K-1 = (1.47 +/- 0.04)M-1 and k(2)= (0.47 +/- 0.01)M-1 s(-1) at 373 K). The experimental activation parameters for the overall reaction were also calculated [E-a = (20.1 +/- 0.9) kcal mol(-1), Delta H-not equal = (19.5 +/- 0.9) kcal mol(-1), Delta S-not equal = (-9.3 +/- 0.5) cal K-1 mol(-1) and Delta G(not equal) = (22 +/- 2) kcal mol(-1)]. The kinetic study together with results on coordination chemistry performed with la and with the BF4 salt of its iridium analogue, [Ir(kappa(2)-P,P-dppe)(2)]BF4 ([1b]BF4), as well as theoretical DFT-calculations allowed us to propose a mechanism for this reaction. The catalytic cycle involves the reversible oxidative addition of formaldehyde to la to yield [Rh(H)(CHO)(kappa(2)-P,P-dppe)(2)](+), followed by the insertion of the olefin into the Rh-H bond to generate [Rh(alkyl)(CHO)(kappa(2)-P,P-dppe)(2)](+), which is considered as the rds of the process [Ea(2) = (23.9 +/- 0.8) kcalmol(-1)]; the cycle is completed by the reductive elimination of the aldehydes to regenerate la and restart the cycle. (C) 2016 Elsevier B.V. All rights reserved.