COMPUTATIONAL EVIDENCE OF THE IMPORTANCE OF SUBSTITUENT BULK ON AGOSTIC INTERACTIONS IN IR(H)(2)((PBU2PH)-BU-T)(2)(+)

While Ir(H)(2)((PBu2Ph)-Bu-t)(2)(+) has been shown experimentally to h ave two agostic Bu-t groups, ab initio B3LYP calculations on IrH2[P(Et )H-2](2)(+) show that the CH3 group of the phosphine ligand does not f orm any agostic bond with the strongly electron-deficient (14-valence electron) metal. In contrast, integrated molecular mechanics/molecular orbital (IMOMM) calculations on the full complex Ir(H)(2)((PBu2Ph)-Bu -t)(2)(+) duplicate the experimentally observed agostic interaction. T hus, at least in this case, the agostic interaction is due in part to the trapping of a C-H bond in the vicinity of the metal by the steric effects of the other groups of the bulky phosphine. This necessity of steric ''constraint'' identifies an additional influential factor for the agostic interaction.