Triphenylphosphine derivatives modified with thiomethyl (SCH3) and methoxy (OCH3) groups in ortho- and para-position of the phenyl ring(s) were screened in situ RH4(CO)(12) catalyzed 1-hexene hydroformylation reaction. The effect of amount and position of the substituents on the hydroformylation results are discussed in terms of geometric and electronic properties of the ligands. The ab initio molecular modeling methods were used to calculate the ground state structures of the free ligands and their higher energy conformers. Cone angle calculations were used to evaluate the steric attributes of the calculated ligand structures. Additionally, study was made of the reactions between the methoxy substituted phosphanes and Rh-2(mu-Cl)(2)(CO)(4). The crystal structure of trans-Rh(CO)Cl(o-OOP)(2) (1) is reported. (C) 2002 Elsevier Science B.V. All rights reserved.