In this work, we have applied Density Functional Theory calculations to investigate the electronic and spacial effects of different phosphorus ligands on the selectivity of the olefin (propene and styrene) insertion reaction into the RhAH bond of the complexes HRh(PR 3 )(CO) 2 (olefin), where the modified ligand PR 3 , is a phosphine (R ¼ H, F, Et, Ph) or phosphite (R ¼ OEt, OPh). M06/SBKJC/cc-pVDZ calculations revealed that the olefin coordination and insertion reaction are dominated by the electronic effects of the phosphorus ligands. A very good correlation between the Tolman electronic factor, v, with the backdonated charges from the metallic center to the olefin and also with the interaction energy of the olefin with the four-coordinated HRh(CO) 2 (PR 3 ) catalyst was found. Using the propene as the substrate and for all the phosphorus ligands investigated, the insertion always proceeds through the reaction path leading to the linear metal-alkyl intermediate. However, when styrene is used, the branched metal-alkyl intermediate is always favored. The structural results obtained for the transition states do not support the existence of a p-allilic intermediates. The regioselectivity obtained for the insertion reaction of styrene results from thermodynamic aspects of the reaction in which the branched metal-alkyl intermediate is always favored by $5 kcal/mol. The M06/SBKJC/cc-pVDZ results are in good agreement with the experimental findings.