The nature of the Rh-diamine precursor in the catalytic cycle of the asymmetric hydride transfer reduction of carbonyl compounds was examined by both theoretical and experimental approaches. On the one hand, calculations based on the DFT theory were performed on various [RhH(NH3)(n)(C2H4)(4-n)] complexes, in the trigonal bipyramidal (TBP) and square pyramidal (SP) forms. Their geometry has been fully optimized, and it was found that the only stable complex corresponds to n = 2 in a TBP form, with the ethylenes in the equatorial plane. On the other hand, mass analyses of the synthesized complexes showed that their composition was [Rh(COD)(diamine)](+) X- (X-= Cl- or PF6-). Several experiments were performed to study the influence of the ligand stoichiometry, the nature of the diene (cyclooctadiene (COD) or others). Finally, the two methods converged and proved that the active species contains only one diamine and one diene bound to the metal.