Ligand effects on the rates of the migratory insertion in rhodium(III) methyl carbonyl complexes

Article abstract of DOI:10.1021/om00058a016

The complexes (η⁵-C₅Me₅)Rh(CO)(X)(Me) (X = I, Br, Cl, MeCO₂, CF₃CO₂) react readily with triphenylphosphine in toluene at 25 °C to yield quantitatively the corresponding acyl species (η⁵-C₅Me₅)-Rh(COMe)(X)(PPh₃). The reactions of the halide complexes are first order in both the substrate and the phosphine. The rate increases in the order I < Br < Cl, following the increasing electronegativity of the halogen atoms. The activation parameters (ΔH;^? = 56 ± 2 (I), 46 ± 3 (Br), 36 ± 2 (Cl) kJ mol^-1; ΔS^? = -79 ± 4 (I), -93 ± 5 (Br), -122 ± 3 (Cl) J K^-1 mol^-1) indicate that the order of reactivity is dominated by changes in ΔH^?. The carboxylate complexes exhibit saturation kinetics, typical of the formation of an intermediate and unprecedented for migratory insertion in a nonpolar solvent. It is proposed that the intermediate is (η⁵-C₅Me₅)Rh(η²-O ₂CR)(COMe) (R = Me, CF₃), where the carboxylate moiety acts as a bidentate ligand to stabilize the otherwise unsaturated species. The rate constant for the formation of the intermediate, determined from the reactions with different nucleophiles, is somewhat larger for X = CF₃CO₂ (k₁ = 0.16 s^-1) than for X = MeCO₂ (k₁ = 0.105 s^-1).