Metal microscopic reversibility

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

Quantitative treatment of rate constants for the hydride attack, / hp, the metal protonation, A fpm. and the exchange process in the framework of Scheme 10.7 have resulted in A hp = 2.7 x 10 M Vs and A fpm = 2.8 x 10 M /s. Thus, the hydride protonation occurs faster by a factor of 10. In earlier chapters we have shown that transition metal hydrides form dihydrogen bonds in the presence of proton donors. Now, based on the principle of microscopic reversibility, one can suggest that proton transfer to a hydridic hydrogen actually occurs via a dihydrogen bond. 

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. 

In addition to their thermodynamic stability, complexes of macrocyclic ligands are also kinetically stable with respect to the loss of metal ion. It is often very difficult (if not impossible) to remove a metal from a macrocyclic complex. Conversely, the principle of microscopic reversibility means that it is equally difficult to form the macrocyclic complexes from a metal ion and the free macrocycle. We saw earlier that it was possible to reduce co-ordinated imine macrocycles to amine macrocyclic complexes in order to remove the nickel from the cyclam complex that is formed, prolonged reaction with hot potassium cyanide solution is needed (Fig. 6-24). 

Given the notion of microscopic reversibility, and the similarity in the active sites of sulfite oxidase and nitrate reductase (assimilatory), determining the mechanism of action of sulfite oxidase impacts upon our understanding of reductases in the (MPT)Mo(0)2 family. A key issue in the mechanism of sulfite oxidase is whether substrate binds to the metal center during the catalytic cycle. Substrate (or product) binding to the molybdenum center, as proposed for the catalytic... 

Since these reactions are influenced by changes in the redox potential of the metal complex, it is possible to change from one process to the microscopic reverse process by changing the ligands attached to the metal. For example, with acetate ligands cobalt(II) is stable with respect to cobalt(III), and, in the presence of bromide ions, cobalt(III) is reduced by alkyl radicals in a ligand transfer oxidation ... 

As already mentioned, the reverse reactions of Fig. 2.6 are reductive elimination reactions. By the principle of microscopic reversibility, the existence of an oxidative addition reaction means that reductive elimination, if it were to take place, would follow the reverse pathway. The reductive elimination of an alkane from a metal-bonded alkyl and hydride ligand in most cases poses a mechanistic problem. This is because clean oxidative addition of an alkane onto a metal center to give a hydrido metal alkyl, such as a reaction like Reaction 2.5, is rare. 

There are a number of modes by which carbon-metal bonds can be cleaved. Conceptually, they can be represented by the microscopic reverse of each of the processes in Equations 2-5 which lead to the alkylation of the metal center. Thus, the reverse of Equation 2 is represented by the well-known electrophilic cleavage of organometals.(5)... 

In each reaction both the oxidation number and the coordination number of the metal increase. Reductive elimination, which is the microscopic reverse of oxidative addition, is a common reaction for removal of a molecule from a metal center. Oxidative addition is far more valuable than reductive elimination for formation of bonds to metals the following sections reflect this. 

Oxidative addition and reductive elimination are the microscopic reverse of each other. In oxidative addition, a metal inserts itself into an X-Y bond (i.e., M + X-Y-> X-M-Y). The X-Y bond is broken, and M-X and M-Y bonds are formed. The reaction is an oxidation, because the metal s oxidation state increases by 2 (and its d electron count decreases by 2), but the metal also increases its total electron count by 2, so it becomes less electron-deficient. The apparent paradox that the oxidation of a metal results in a larger electron count is an artifact of the language with which compounds are described. 

Reductive elimination (X-M-Y — M + X-Y) is the microscopic reverse of oxidative addition. This reaction is usually most facile when the X-Y bond is strong (e.g., H-Ti-Bu —> Bu-H + Ti). Not as much is known about the mechanism of reductive elimination as is known about oxidative addition. It is known that the two groups must be adjacent to each other in the metal s coordination sphere. In square planar Pd complexes ((R3P)2PdR2), if the PR3 groups are forced to be trans... 

Insertions and (3-eliminations are also the microscopic reverse of each other. In an insertion, an A=B 77 bond inserts into an M-X bond (M-X + A=B —> M-A-B-X). The M-X and A=B bonds are broken, and M-A and B-X bonds are formed. Insertion is usually preceded by coordination of the A=B 77 bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M-X bond is replaced with an M-A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a 77 bond. The nature of the reaction requires that the new C-M and C-H bonds form to the same face of the A=B 77 bond, resulting in syn addition. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

Elimination is the microscopic reverse of insertion. Just as insertion does not, a / -elimination causes no change in the oxidation state, d electron count, or total electron count of the metal. By far the most common /3-elimination is the (3-hydride elimination, in which M-A-B-H -h> M-H + A=B. The /3-hydride elimination is the bane of the organometallic chemist s existence, as it causes many metal-alkyl bonds to be extremely labile. /3-Alkoxy and /3-halide eliminations are also known, as in the reaction of BrCH2CH2Br with Mg. 

Hydrogen atom transfer from the hydride form of the catalyst to monomer (eq 15) is relatively unexplored in comparison with the initial reaction in the catalytic cycle, hydrogen atom abstraction from the growing radical. This is despite the fact that the two reactions are essentially the microscopic reverse, because the substituents on the organic fragment are relatively removed from the metal center. Early investigations of CCT were frustrated by the fact that concentrations of LCoH were below detection lim-... 

Watson et al. reported a leading example of (3-carbon elimination observed with a well-defined metal complex [67]. Thermal decomposition of a lutetium-isobutyl complex having a vacant coordination site leads to the formation of a lutetium-methyl complex and propene by way of (3-methyl elimination, the microscopic reverse of olefin insertion. A concerted four-center transition state is proposed. This study demonstrated that (3-carbon elimination is an energetically accessible process, and provided a model for the chain transfer that occurs during propene oligomerization. 

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