Dissociative mechanism results

Nickel carbonyl has been shown to have a very high retention— 98.7%—both in the pure hquid and as 10% solution in n-heptane. It was argued that this represents the results following essentially complete isotopic exchange. Since the exchange of CO with Ni(CO)4 is known to occur quite rapidly by a dissociation mechanism, reformation of nickel carbonyl following the nuclear reaction would proceed rapidly by the reaction... 

The values appear reasonable and, in contrast to the interpretation of the nmr data, (which may possibly relate to a different process) the results can be explained by a dissociative mechanism. 

An attempt has been made to predict the general requirements for the operation of a non-dissociative mechanism of tautomerisation between oxygen and nitrogen centres (Bensaude et al., 1977). The proximity of the two centres between which the proton is transferred is obviously of critical importance but more experimental results are required before a clear picture will emerge. 

Table XII contains the enthalpy and free-energy differences of the critical intermediate species for the anthraphos Rh catalyst. Although no experimental results are available yet, our predicted energies show a much smaller dissociation energy for H2 loss, the first step in the dissociative mechanism (8 is more stable by 13 kcal/mol relative to 4). In contrast, the oxidative addition intermediate of the first step in the associative mechanism, the M(V) species 5, is 14 kcal/mol less stable...

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

Kinetic studies on the thermal racemization of the sulfonium salt 211 revealed that the process requires much lower activation energies when compared with thermal racemization of sulfoxides. Comparison of the relative rates for racemization of sulfonium salts 211, 212, and 213 was taken (151) as evidence that racemization of 211 is the result of pyramidal inversion, not of an alternative dissociation mechanism. On the other hand, Brower and Wu (249) concluded that the volume of activation for the racemization of sulfonium ion 211, AF = +6.4 ml/mol, is more compatible with a transition state in which partial dissociation has occurred. 

It is not surprising that the substitution processes on seven-coordinate 3d metal ions follow an interchange, rather than a limiting dissociative mechanism. In the case of seven-coordinate Fe(III) complex we even found an associative interchange mechanism for the substitution of solvent molecules as a result of the high n-acceptor ability of the fully conjugated pentadentate ligand system present in its equatorial plane 44). 

Solvolysis studies of meta- and para-substituted phenyl phosphates (240) in anhydrous Bu OH and in Am OH have revealed that generally reactions of dianions are much faster in alcohols than in water. For example, the dianion of p-nitrophenyl phosphate (240 X = 4-NO2) reacts 7500- and 8750-fold faster in Bu OH and Am OH, respectively, than in water." The results of a theoretical study of the reactivity of phosphate monoester anions in aqueous solution do not support the generally accepted view that Brpnsted coefficients fhg = —1.23 and jSnuc = 0.13 determined more than 30 years ago for the uncatalysed reaction of water and a monophosphate dianion (241) represent conclusive evidence for the dissociative mechanism. It is suggested that, instead, the observed LFERs could correspond to a late transition state in the associative mechanism." ... 

Although there are numerous investigations of concerted dissociative ET reported, particularly those involving alkyl and benzyl halides, examples of other molecular systems that undergo putative concerted dissociative ET are not as well documented. Peroxides and endoperoxides are another class of compounds where ET to the oxygen—oxygen bond has recently been shown conclusively to follow a concerted dissociative mechanism. The concerted nature of the dissociative ET is, in part, a result of the very weak 0—0 bond, which is a necessary condition, as illustrated by equation (41). 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

Cleavage in aqueous solution in the presence of other anions, Y-, has been studied for Y- = N03-, SCN-, Cl-, and CH3S03 (355-358). In all these reactions variable amount of Co(NH3)5Y2+ are produced together with the aqua complex, and extensive competition studies have been reported for the uncatalyzed as well as for the acid-catalyzed cleavage reaction. The results of these competition experiments are in keeping with an essentially dissociative mechanism for bridge cleavage. 

At H+ concentrations higher than about 0.2 M,Eq.(ll) does not describe the dependence of the dissociation rates, since kp exhibits saturation behavior [47, 48,50]. At these higher H+ concentrations, the monoprotonated complexes LnHL predominate, because the protonation constant values (KLnHL) are -20-200 (Table 3). The saturation behavior of the [H+] vs. kp plots is presumably a result of the accumulation of the protonated complexes. On the basis of the rate law derived and the experimental data obtained in more acidic solutions, the assumed acid-catalyzed dissociation mechanism can be depicted in a reaction scheme [50] ... 

The three-phase test has been used to distinguish between associative and dissociative mechanisms. Thus, in this case the negative results in the trapping of the intermediate with dienic polymers seem to support a stepwise mechanism in the formation of cyclocondensation adducts with siloxydienes and l-azetin-4-one. 

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