Dissociative intermediate competition studies

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

Chelating ligands have a much lower propensity for dissociation. Yet detailed kinetic studies on similar systems with bidentate phosphine ligands, /ac-(L2)PtMe3X (L2 = dppe (bis(diphenylphosphino)ethane), dppbz (bis(diphenylphosphinobenzene) X = I, OAc, OPh) also showed that ligand dissociation was required prior to any C-C coupling (48-51). In this case, however, the X- group rather than the phosphine was lost to form a five-coordinate intermediate, as shown in Scheme 11. A competitive C-X reductive elimination also occurs from these complexes and involves the same five-coordinate cation (Section V. A). 

The base catalyzed solvolysis and substitution reactions of cobalt(III) complexes has provided some of the most fertile ground for the study of dissociatively activated processes and the attempts to distinguish between and D mechanisms in terms of the lifetime of the intermediate species. The dissociative intimate mechanism has been demonstrated by a variety of trapping, competition and stereochemical studies, The base catalyzed substitutions of [Co(NH3)5X]"+, in which addition of small amounts of OH in the presence of excess leads to a mixture of [Co(NH3)5Y] and [Co(NH3)50H], are all consistent with a dissociatively activated process in which there is competition between Y and H2O for the place vacated by At the first approximation, the ratio... 

As noted above, one of the first indications that photogenerated "unsaturated" metal carbonyls such as Cr(CO)5 were indeed solvent coordinated was the demonstration in flash photolysis experiments that the rates of the back reactions with CO as well as reactions with other other ligands are markedly dependent on the nature of the solvent medium. For example the second order rate constant k2 for eq. 5 was reported to be 3.6 x 10 M s in cyclohexane[34] and 3 x 10 M s in perfluoro-methylcyclohexane [25]. The reaction kinetics are second order, for the large part, the substitution mechanisms of these intermediates reacting with CO or other substrates are not yet fully elucidated. However, recent kinetics studies by Dobson and coworkers [35] of the reaction of Cr(CO)5S with S = n-heptane or chlorobenzene with 1-hexene or piperidine as a trapping agent have led to the conclusion that the substitution reactions occurred via competitive dissociative and interchange pathways. Complementary studies... 

Kinetic studies of this process do indeed show the catalysis being first order in the concentration of the ruthenium cluster and inversely dependent on the partial pressure of carbon monoxide indicating thus that the most important intermediate in the reaction is the coordinately unsaturated species H4Ru4(CO)ii. Coordinately unsaturated sites in the catalyst would be achieved, in this case, by the dissociation of a CO-ligand. The catalytic effect of the unsaturated intermediate depends on the reversible insertion of ethylene into the Ru-H bond. However there is competition between this insertion and the addition of carbon monoxide or H2 for obtaining the starting compound or the hexahydride H6Ru4(CO)n respectively. 

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