KINETICS AND MECHANISM OF CO LIGAND SUBSTITUTION IN THE RING-SUBSTITUTED INDENYL RHODIUM COMPLEXES [(ETA-5-C9RNH7-N)RH(CO)2]

Effects of controlled electronics and sterics at the metal centre on the kinetics of associative (SN2)CO substitutions by a number of phosphine and phosphite ligands in a series of indenyl ring-substituted rhodium bis(carbonyl) complexes, [(eta-C9RnH7-n)Rh(CO)2] (R = Cl, CH3, OCH3, OCH2O, C4H9; n = 1-7), are reported. The reaction is first order in the indenyl complex and first order in the incoming nucleophile. Comparison of the molecular structure of [(eta-4,5,6,7-Me4C9H3)Rh(CO)2] with that of the unsubstituted analogue shows that although the steric bulk of the indenyl ligand is increased by alkylation, there is no effect on the ground state structure of these molecules. Rates of CO substitution are dependent on both the nature of incoming nucleophile and the rhodium complex (electron richness of the metal centre, perturbations to the lowest unoccupied molecular orbital (LUMO), and the steric demands of the indenyl ligand). An alternative eta-5- to eta-3-exo- to eta-5-indenyl ligand slippage mechanism is suggested as being one plausible explanation of the kinetic results.