The solution structures of the dominant species present during the styrene hydroformylation using rhodium precursors [Rh(µ-OMe)(cod)] 2 (1) and the dithiolate-bridged complex [Rh 2 (µ-BCOS)(cod) 2 ] 2 (9) in the presence of BDPP [(2S,4S)-bis(diphenylphosphine)pentane] have been determined. The high-pressure spectroscopic studies have revealed that the mononuclear complex [HRh(BDPP)(CO) 2 ] ( 7) is the predominant species during the hydroformylation process, but the dimeric species [Rh(BDPP)(CO) 2 ] 2 ( 8) is also present and in equilibrium with 7. Equilibrium constants were measured by 31 P NMR spectroscopy (at [Rh] ) 0.0138-0.0157 grat‚L -1 , 5-50 bar of H 2 , 5 bar of CO, 298-343 K, the molar ratio of 7:8 ranges from 1 to 20). Kinetic measurements have shown that the half-life of the species at ambient temperature is on the order of magnitude of 1 min. Variable-temperature 31 P and 1 H NMR spectra (293-193 K) revealed fluxional behavior in complex 7 where the bidentate diphosphine BDPP adopts equatorial-axial coordination to rhodium in the trigonalbipyramidal complex [HRh(BDPP)(CO) 2 ] (bite angle ≈90°). A low free energy of activation (∆G q 193K ) 44 kJ‚mol -1 and ∆G q 293K ) 44 kJ‚mol -1 ) for phosphorus exchange has been measured for the fluxional process observed in complex 7.