LIGAND EFFECTS ON THE RATES OF THE MIGRATORY INSERTION IN RHODIUM(III) METHYL CARBONYL-COMPLEXES

The complexes (eta-5-C5Me5)Rh(CO)(X)(Me) (X = I, Br, Cl, MeCO2, CF3CO2) react readily with triphenylphosphine in toluene at 25-degrees-C to yield quantitatively the corresponding acyl species (eta-5-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 (DELTA-H double-ended-dagger = 56 +/- 2 (I), 46 +/- 3 (Br), 36 +/- 2 (Cl) kJ mol-1; DELTA-S double-ended-dagger = -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 DELTA-H double-ended-dagger. 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 (eta-5-C5Me5)Rh(eta-2-O2CR)(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).