Solvation intermediates

At an early stage, only for diphosphine ligands capable of trans-coordination were detected hydride solvate intermediates [71], but a catalytic pathway based on diphosphine trans-chelation has not been developed to date (see [80b]). 

The significance of these solvation intermediates lies in their relationship to intermediates along the growth pathway to internally doped nanocrystals, since these data reveal the thermodynamic stability of tetrahedral surface-bound Co2+ ions. Binding of impurity ions to nanocrystal surfaces is a necessary step in doping a growing nanocrystal. The absence of a detectable intermediate between... 

The main problem with predictions of kinetic preferences based on force-field calculations of relative stabilities of reaction intermediates arises from the fact that the energies of the competing intermediate structures usually differ by amounts smaller than the accuracy of the energy calculations. These calculations are inherently inexact, since the force-field parameters for the intermediates are mostly unknown (high-level ab initio calculations would be needed to determine them). Moreover, the probability that a reaction passes via a given intermediate depends not only on its enthalpy, but also on entropy thus, a dynamic description of the solvated intermediate would be required. Finally, even if we knew the structure of a reaction intermediate perfectly, it would always remain an approximation for the geometry of the transition state, whose free energy is the real determinant of the reaction kinetics. 

Figure 15 Nanosecond kinetics (solid lines) of (a) j5-CpMn(CO)3 and (b) j5-CpRe(CO)3 in neat room-temperature triethylsilane after 295 nm photolysis for the ethyl-solvate intermediates (the decaying traces) and the final products (the rising traces). (Adapted from Ref. 45.)... 

The results of such an analysis are also given in Table 8. Similarly to the previously obtained results, they too show that the less solvated intermediates are formed in the course of the electrode reaction. Relatively good correlation coefficients may suggest that the mechanisms of a given reaction in various solvents are similar. However, in view of the complexicity of such reactions and the influence of the doublelayer structure on their rate, this suggestion should be considered with caution. 

Besides the dissolving power of a solvent toward substrate and supporting electrolyte, the ability to solvate intermediate cations and anions is also important. The chemical properties of charged species are dependent on whether there are formed tight ion pairs, solvent-separated ion pairs, or symmetrically solvated ions. Redox potentials of ions are... 

Dissociative processes were also implicated in HCl cleavage reactions of ci5-[PtMeR(PEt3)2] (R = methyl, phenyl, or mesityl) and their subsequent isomeri-zations. The cleavage mechanism is believed to involve a fast preequilibrium between substrate and chloride, combined with a parallel protonation of both platinum species, where the proton attacks the Pt—Me bond (Scheme 3). The spontaneous isomerization of the solvated intermediates ci5-[PtR(S)(PEt3)2] was followed either by P NMR or UV/visible spectroscopy. Dissociative loss of the solvent molecule, S, was implicated because of the relative insensitivity of the reaction rate to the steric bulk of R, and because of the positive values of AS. ... 

About 75% of the bleached absorptions recover within 200 ps, which shows that 25% loss of the parent molecules were driven to the CO loss channel, since no long-lived non-dissociative excited states were present. This result was consistent with earlier quantum yield measurements. The naked 16-valence-electron complex Rh(Cp)(CO) was not directly observed. The early time ps-TRIR spectra show broad featureless transient absorptions due to the formation of electronically and vibrationally excited states. On the nanosecond timescale, Rh(Cp)(CO)(alkane) converts to Rh(Cp)(CO)(alkyl)H. We have recently repeated these experiments and these data are shown in Figure 9, from which the rate of C-H activation from the solvated intermediates can be measured. 

The mechanism of the photolytic reaction of Mn(7r-C5H5)(CO)3 with TT-interacting ligands such as indene or maleic anhydride depends on the solvent, in that in THF there appears to be a solvated intermediate Mn(7T-C6H6)(CO)2(THF), whereas in the poorer solvents n-hexane or benzene the intermediate is simply Mn(7r-C5H5)(CO)2. ... 

It is easy to imagine that, because it is cationic, the solvated intermediate would be much more susceptible to Br attack than the starting complex. Because the solvent concentration cannot normally be varied without introducing rate changes due to solvent effects, the [solv] term does not usually appear in the experimental rate equation, which therefore has the form... 

Under conditions where (A 2/ 3[Cl-])>[py],/robs reduces to Arobs = k + Ar [py], the latter expression having the same form as the rate law normally found for substitution at platinum(n) but with a very different meaning in this case. In the current period the reaction has been examined for a series of substituted pyridines, and shows the same kinetics. The competition ratio for the solvated intermediate, kjk2 is found to be relatively unaffected for 3-, 4-, and 5-substituted pyridines but is considerably lower in the case of pyridines having substituents in the ortho position. This is quite clearly a steric effect rather than a consequence of any differences in nucleophilicity. A more detailed examination of the k path (R = Ph, n-tolyl, or mesityl) shows, in addition, a rate-retarding effect which derives from the presence of ortho substituents on the trans ligand R. Retardation by ortho substituents either on... 

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