ALKYNE LIGAND ENHANCEMENT OF THE SUBSTITUTION LABILITY OF MONONUCLEAROSMIUM, RUTHENIUM, AND IRON CARBONYLS

The kinetic influence of an alkyne ligand, hexafluorobut-2-yne (HFB), has been investigated by studying the reactions of phosphines (PR3) wi th the complexes M(CO)(4)(eta(2)-HFB) (M = Fe, Ru, Os). The rate of pr oduction of M(CO)(3)(PR3)(eta(2)-HFB) is independent of the nature and concentration of the phosphine in all cases, indicating that the rate -controlling step is CO dissociation. The kinetic parameters, k(1) (s( -1), 25 degrees C), Delta H (kJ mol(-1)), and Delta S* (cal mol(-1) K -1) are: 9.5, 88.2 +/- 2.3, 70 +/- 10 (Fe); 1.25 x 10(-2), 103.6 +/- 2 .4, 66 +/- 8.6 (Ru); 3.5 x 10(-3), 99.5 +/- 0.8, 21 +/- 2.7 (Os). When the rate constants at 25 degrees C for M(CO)(4)(eta(2)-HFB) are compa red to those of the parent M(CO)(5), the ratios are similar to 3 x 10( 13), 1.8 x 10(2) and 1 x 10(7) for M = Fe, Ru, and Os, respectively. C learly the alkyne increases the substitution lability, and the effect is spectacular with Fe, very large with Os, and substantial but relati vely more modest with Ru, The increased lability results mainly from a reduced Delta H of similar to 80, 10, and 33 kJ mol(-1) for Fe, Ru, and Os, respectively, and this is attributed largely to stabilization of the transition state by 4-electron donation from the alkyne ligand. Also reported are kinetics of formation of some trans M(CO)(2)(PR3)(2 )(eta(2)-HFB) complexes and an extension of earlier work on the Os(CO) (5)/PPh3 system.