A Density Functional Study of β-Hydride and Methyl Migratory Insertion in CpM(PH3)(CH2CH2)R+(M = Co, Rh, Ir; R = H, CH3)

The hydride and methyl β-migratory insertion processes in CpM(PH 3 )(CH 2 CH 2 )R + (M = Co, Rh, Ir; R = H, CH 3 ) as well as the microscopically reverse β-elimination reactions have been studied by relativistic density functional theory. The calculations reveal that the β-migratory insertion reactions of the olefin hydride complexes CpM(PH 3 )(CH 2 CH 2 )H + (M = Co, Rh, Ir) have electronic reaction barriers of 0.3 (Co), 2.7 (Rh), and 6.1 kcal/mol (Ir), respectively. Further, the β-migratory insertion reactions of hydride are exothermic for cobalt (Δ H e = −3.4 kcal/mol) and rhodium (Δ H e = −1.0 kcal/mol), but endothermic for iridium (Δ H e = 3.7 kcal/mol). Relativistic effects are important for the calculated trends within the cobalt triad. Without relativity the β-migratory insertion reactions would be exothermic for all three metals. For the corresponding β-migratory insertion reactions of methyl the barriers are 15.2 (Co), 19.8 (Rh), and 23.2 kcal/mol (Ir), respectively. The reactions are exothermic for all three metals with Δ H e = −12.7 (Co), −8.5 (Rh), and −5.3 kcal/mol (Ir), respectively. Structures of reactants, transition states, and products were fully optimized. For the hydride migration, the transition states are close to the hydrido olefin systems CpM(PH 3 )(CH 2 CH 2 )H + for M = Co and Rh, whereas the transition states for the iridium hydride resemble the ethyl compound CpIr(PH 3 )(CH 2 CH 2 H) + . The transition states for the methyl migration are product-like for all three metals. The most stable conformation of the ethyl and propyl product complexes CpM(PH 3 )(CH 2 CH 2 R) + exhibits in all cases a β-agostic M−H−C interaction. The strength of this interaction decreases down the cobalt triad. An extensive thermochemical analysis is provided for the relative stability of CpM(PH 3 )(CH 2 CH 2 )R + and CpM(PH 3 )(CH 2 CH 2 R) + (M = Co, Rh, Ir; R = H, CH 3 ).