Electronic effects on rhodium-catalyzed hydroformylation of l-hexene with electronically dissymmetric DIPHOS derivatives [3,5-(CF3)(2)C6H3](2)PCH2CH2PPh2 = [DIPHOS-(3,5-CF3,H)] (1), [2-(CF3)C6H4](2)-PCH2CH2PPh2 = [DIPHOS-(2-CF3,H)] (2), [3,5-(CF3)(2)C6H3](2)PCH2CH2P[2-(CH3)C6H4](2) = [DIPHOS-(3,5-CF3,2-CH3)] (3), and [2(CF3)C6H4](2)P[2-(CH3)C6H4](2) = [DIPHOS-(2-CF3,2-CH3)] (4) were investigated. Two apical-equatorial chelate isomers were observed for model (diphosphine)Ir(CO)(2)H complexes of dissymmetric diphosphines 1-4. In each case, the equatorial phosphine of the major isomer (96-60%) had electron-withdrawing aryl substituents. These dissymmetric DIPHOS derivatives were used to test the hypothesis that an electron-withdrawing substituent on an equatorial phosphine increases the hydroformylation n:i ratio while an electron-withdrawing substituent on an apical phosphine decreases the n:i ratio. In agreement with the predictions of this hypothesis, hydroformylation with the dissymmetric diphosphine ligand DIPHOS-(3,5-CF3,H) (1), gave an n:i ratio of 4.2:1, higher than either of the symmetric ligands DIPHOS, 2.6:1, and DIPHOS-(3,5-CF3), 1.3:1. Similar observations were made for hydroformylations with 2-4.