Reaction Dynamics of the Tricoordinate Intermediates MCl(PPh3)2 (M = Rh, or Ir) as Probed by the Flash Photolysis of the Carbonyls MCl(CO)(PPh3)2

Article abstract of DOI:10.1021/ja00236a022

Reported is a kinetics flash photolysis investigation of the rhodium(I) complex RhCl(CO)(PPh3)2 in benzene solution.These results are interpreted in terms of the transient formation of the unsaturated species RhCl(PPh3)2 (A), an intermediate crucial to proposed mechanisms of Wilkinson's catalyst reactions such as olefin hydrogenation, but which has not been the subject of previous direct investigation.Kinetics of the dimerization of A and of the reactions of this transient with CO, C2H4, PPh3, and H2 are also described.The second-order rate constant for the reaction with H2 is 1.0E5/M.s in good agreement with the value >7E4/M.s estimatted by Halpern and Wong (J.Chem.Soc., Chem.Commun. 1973, 629) from kinetics investigations of the hydrogenation of RhCl(PPh3)3.Furthermore, the equilibrium constant for triphenylphosphine dissociation from RhCl(PPh3)3 can be calculated as 2.3E-7 M from the ratio of the first-order dissociation rate constant 0.68/s determined by those workers and the second-order rate constant 3.0E6/M.s for the back reaction determined here.Rates of the subsequent reactions of other adducts formed from A and various ligands with the CO liberated in the flash experiment were also determined.Flash photolysis studies of the iridium(I) analogue IrCl(CO)(PPh3)2 in benzene demonstrated CO photolabilization in this case as well.The back reaction of the resulting transient species IrCl(PPh3)2 with CO displayed second-order kinetics with the respective rate constant 2.7E8/M.s.These results are discussed in terms of the catalytic mechanisms involving such species.