SUBSTITUTION KINETICS OF SOME TETRACARBONYL(ETA(2)-ALKENE)OSMIUM COMPLEXES

The kinetics of displacement of the alkene from Os(CO)(4)(eta(2)-alkene) by P(OEt)(3) or PPh(3) in heptane or dodecane have been studied (alkene=ethene, 1-octene, methyl acrylate). The reactions all proceed by dissociative paths with positive values of Delta S* (60-80 J K-1 mol(-1)) which contrast with the very small value 5.6 +/- 0.1 J K-1 mol(-1) (recalculated as -2 +/- 10 J K-1 mol(-1)) reported elsewhere for dissociative loss of CO from Os(CO)(5). Substantial amounts of the disubstituted products Os(CO)(3)L(2) are produced by the reactions in addition to the expected Os(CO)(4)L, a phenomenon also observed after similar reactions of Fe(CO)(4)(eta(2)-alkene) complexes although the reasons for this seem to be different in the two cases. Comparison with data obtained elsewhere shows that the pronounced 'triad effect' exhibited by the M(CO)(4)(eta(2)-methyl acrylate) complexes (M=Fe, Ru, Os) is caused entirely by the relative values of Delta H* (152 +/- 2, 116 +/- 2, 149 +/- 2 kJ mol(-1), respectively). The relationship between the activation enthalpies for alkene dissociation and the intrinsic strengths of the metal-(eta(2)-alkene) bonds is briefly discussed. What appears to be the complex Os(CO)(4)(eta(2)-vinyl acetate) behaves quite differently from its methyl acrylate isomer.