The electronic effects of electron withdrawing aryl substituents on equatorial and apical diphosphines were investigated. Chelating diphosphines designed to coordinate in diequatorial or in apical-equatorial positions were synthesized, and their effects on the regioselectivity of rhodium catalyzed 1-hexene hydroformylation were observed. Only diequatorial coordination was observed for 2,2'-bis[(diphenylphosphino)methyl]-1,1'-biphenyl (BISBI) complexes (BISBI)Ir(CO)(2)H (8) and [BISBI-(3,5-CF3)]Ir(CO)(2)H (10), and only apical-equatorial coordination was seen for 1,2-bis(diphenylphosphino)ethane (DIPHOS) complexes (DIPHOS)Ir(CO)(2)H (14) and [DIPHOS-(3,5-CF3)]Ir(CO)(2)H (15). For the trans-1,2-bis [(diphenylphosphino)methyl] cyclopropane (T-BDCP) complexes, a mixture of diequatorial and apical-equatorial complexes was seen. For (T-BDCP)Ir(CO)(2)H (12), 12-ee was favored over 12-ee by 63:37, but for [T-BDCP-(3,5-CF3)]Ir(CO)(2)H (13) the conformational preference was reversed and a 10:90 ratio of 13-ee:13-ee was seen. The electron withdrawing groups in the equatorial positions of BISBI-(3,5-CF3) (1) and T-BDCP-(3,5-CF3) (2) led to an increase in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation. However, electron withdrawing aryl substituents in the apical positions of DIPHOS-(3,5-CF3) (3) led to a decrease in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation.