The electronic effect in the rhodium diphosphine catalyzed hydroformylation was investigated. A series of electronically modified thixantphos ligands was synthesized, and their effects on coordination chemistry and catalytic performance were studied. Phosphine basicity was varied by using p-(CH3)(2)N, p-CH3O, p-H, p-F, p-Cl, orp-CF3 substituents on the diphenylphosphine moieties. X-ray crystal structure determinations of the complexes (thixantphos)Rh(CO)H(PPh3) and (p-CH3O-thixantphos)Rh(CO)H(PPh3) were obtained. The solutions structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)(2)H complexes were studied by IR and NMR spectroscopy. IR and H-1 NMR spectroscopy showed that the (diphosphine)Rh(CO)(2)H complexes consist of dynamic equilibria of diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium compositions proved to be dependent on phosphine basicity; the ee:ea isomer ratio shifts gradually from almost one for the p-(CH3)(2)N-substituted ligand to more than nine for the p-CF3-substituted ligand. Assignments of bands to ee and ea isomers and the shifts in wavenumbers in the IR spectra were supported by calculations on (PH3)(2)Rh(CO)(2)H, (PH3)(2)Rh(CO)(2)D, and (PF3)(2)Rh(CO)(2)H complexes using density functional theory. In the hydroformylation of 1-octene and styrene an increase in 1:b ratio and activity was observed with decreasing phosphine basicity. Most remarkably for 1-octene the selectivity for linear aldehyde formation was between 92 and 93% for all ligands. These results indicate that the chelation mode in the (diphosphine)Rh(CO)(2)H complexes per se is not the key parameter controlling the regioselectivity. Mechanistic explanations of the effect of the natural bite angle on regioselectivity are reconsidered.