Insertion thermodynamic factors

Migratory insertion of CO into a transition metal-hydride bond is not common. Nevertheless, its importance in catalytic reactions such as the Fisher-Tropsch synthesis has attracted much attention T The marked difference in behavior between metal alkyls and metal hydrides with respect to the insertion of CO is largely a consequence of thermodynamic factors ... 

Inserting (9.16b) in the expression for F yields the equation for the thermodynamic factor in the case of Langmuir adsorption ... 

Insertion processes are reversible in certain cases, subject to thermodynamic factors in Eq. 7.1. Particularly important among the deinsertion processes is the decarbonylation by which a compound with one less carbon unit is produced. De-carbonylation of acyl halides and aldehydes are utilized for removing a carbonyl... 

Following convention in the literature, we have changed here from a molal scale to a fractioncil occupancy scale, defined by n, = p./ + v/iT Inyj , where = cjcs,minsertion compounds, the thermodynamic factor is related to the open-circuit potential by... 

In this section we present the advantage of the above methods involving only semiinfinite diffusion for the determination of both the chemical diffusion coefficient and the enhancement (or thermodynamic) factor of M0O3 electrode materials inserted by Li" ions. M0O3 crystals and films are considered [15, 16]. 

One common feature of all M + hydrocarbon systems mentioned in Sec. 1.2.2 is that none of the products resulted from cleavage of a C-C bond. This is a result of several factors. First, C-H bonds are less directional than C-C bonds (Sec. 1.1), allowing for multicentered bonding at the transition state, which tends to lower the barrier for C-H insertion relative to C-C insertion.2,18,22 Second, since M-H bonds are usually stronger than M-C bonds, intermediates resulting from insertion into a C-H bond are usually thermodynamically favored.141 Third, there are typically more C-H bonds in hydrocarbons than C-C bonds, so C-H insertion is also statistically favored. Finally, C-H bonds are more accessible to an incoming metal atom and are therefore more susceptible to insertion. 

The overall enthalpy change of the insertion process contains contributions from four bonds (M-CO, M-COR, M-R and CO-R). As there is no significant difference between (Mn-R) and Zs(Mn-COR) then, at least in the case of manganese and hydrocarbon groups, R, the dominant factor will be the difference between T (Mn-CO) and E R-COX) [for R = CH3, E = 339 kJ mop1 (X = H), 370 kJ mol"1 (X = Cl) (Ref.23 )] which suggests that the insertion reaction is thermodynamically favoured with respect to decarbonylation. Kinetic studies of the carbonyl insertion reaction in solution have shown87) that the enthalpy of activation is 62 kj mol-1 for inser-... 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

The possible factors involved in the biological selectivity towards metal ions have been considered by Frausto da Silva and Williams3 and by Kustin et al.4 In terms of thermodynamic selectivity a useful formalism for the uptake of any metal ion from a multimetal system is the quotient A Cm, where Km is a relative stability constant and Cm is the concentration of the metal ion. However, as these authors point out,3 a combination of both thermodynamic and kinetic properties must be considered. An appreciation of kinetic factors is often absent in this field, but must be of prime consideration in chelate exchange reactions and in the final irreversible step of metal ion insertion to form the metalloenzymes. 

IfK2, kl9 k u and k2 had the same values as K 2, k u k l9 and k 2, then the optical purity of the product, RH, would be determined solely by the value of the diastereomeric equilibrium constant Kv If, however, the primed and unprimed constants were different, the final optical yield could be determined by both thermodynamic and kinetic factors, and in one extreme could result in the observation that the preferred enantiomer of the product originated in the minor dia-stereomer. Clearly, kinetic factors can be important since the steric interactions of the initial two diastereomers are different and these could affect the rate constants of the reaction. Moreover, the o--alkyl intermediate is chiral, as shown for one of the initial olefin diastereomers in Figure 4, and the rate of hydrogen addition and insertion... 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

In contrast to the biscyclopentadienyl derivatives of vanadium(III), those of nio-bium(III) do not normally undergo carbon monoxide insertion. The methyl derivative of biscyclopentadienylniobium(III), Nb( -C5H5)2Me(CO), does not react with CO even under pressure (200 atm) at 20-50 C in toluene solution. Failure to observe this reaction is due to thermodynamic rather than kinetic factors, since methylation of [Nb(f) -C5H5)2(CO)2] with LiMe gave the following methyl derivative ... 

Inhibition of propene insertion by ether or monomer 7A is mild but kinetically important at the concentrations used for NMR experiments ( 0.05 M), slowing the rates by a factor of 20-30. Inhibition is very pronounced with the stronger ligands Al(0113)3 and THF. This kinetic inhibition is due to the thermodynamic stability of the adducts (see Figure 4). In fact, complexation of 7L by both Al (0112)3 slows the propene... 

Transition metal complexes usually favor insertion into the carbon-sulfur bond adjacent to C(alkenyl)-S bond (type a), but some complexes insert into the C(aryl)-S bond (type b) [117]. Ab initio calculations suggest that the most important controlling factor for determining selectivity in the C-S bond cleavage of benzothiophene is the thermodynamic stability of the resulting metal-carbon bond [118],... 

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

Equation (2.23) is completely analogous to Eq. (2.21), but there is some important difference. It is possible to conduct a thermodynamic process in which dU = 0 and Ti T2. Setting d5i + A.d52 = 0, we get from the first equality in Eq. (2.23) 71d5i + T2d52 = 0, X = T2/T1. Inserting this into the second equality ofEq. (2.23), we get the identity T = Ti. Therefore, it is necessary to obtain the factor A. from an independent relation, if we want to calculate some specific values of 5i and S2 where the energy would be stationary. However, we can state that for stationary energy df7 = 0, Ti -> T) implies d5i -d52, which means the conservation of entropy. ... 

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