Metal alkyls bond energies

Carbanionic metal alkyls and hydridic metal hydrides will react with alcohols or phenols to give alkoxides and phenoxides, typically in excellent yields. The reaction is also important as it forms the basis for the calorimetric measurement of a large number of metal-alkyl bond dissociation energies.93,94 This synthetic method tends to be very convenient due to the volatility of the generated alkane or hydrogen side products. Monoalkyl alkoxides of Be,95 Mg96 and Zn97 can be obtained in this way (equation 26). 

Hence, the value of -AG(por)Fe Bu. is between 95 and 142 kJ mol-1 (117 25 kJ mol-1, Table 13.9), and depends to some degree on the electron density of the porphyrin ring. The data of Tables 13.6 and 13.7 have been used to estimate the metal-carbon bond energies for various alkyl groups via similar calculations [Eqs. (13.24)-(13.29)], and are summarized in Table 13.9. 

This chapter addresses the following themes The estimation of the metal-alkyl bond dissociation energies of coenzyme B12 and related compounds. The factors that influence metal-alkyl bond dissociation energies of organocobalamins and related compounds. Reactions between free radicals and metal complexes. 

The third theme is of significance in this context, not only because the proposed mechanism of coenzyme Bl2-dependent rearrangements (Equations 2 and 3) involves the generation of organic free radicals in the presence of a metal complex (i.e., vitamin Bi2r), but also because of the potential utility of metal complexes as radical traps in studies involving kinetic approaches to the estimation of metal-alkyl bond dissociation energies. 

Reaction 12 is the reverse of the metal-alkyl bond dissociation process (Equation 15). Hence, the activation enthalpies (AH ) of such homolytic bond dissociation reactions are expected to be close to the corresponding bond-dissociation energies. 

As elaborated below, the facile occurrence of Reactions 13 and 14 is important in this context because of the potential usefulness of these reactions as radical trapping processes in kinetic approaches to the determination of metal-alkyl bond dissociation energies. 

Approaches to the Estimation of Metal-Alkyl Bond-Dissociation Energies... 

Few transition-metal-alkyl bond-dissociation energies are known reliably (16). Potential approaches to the estimation of such dissociation energies encompass the following ... 

Photochemical. Determination of the threshold wave length for the photolytic dissociation of a metal-alkyl bond yields an upper limit for the corresponding thermal bond dissociation energy (5, 19), but the assumption that the photochemical threshold approximates the bond dissociation energy does not appear to be warranted. 

Among these alternative approaches, 1 and 2 are considered the most promising for the determination of metal-alkyl bond-dissociation energies in coenzyme Bi2 and related compounds. The application of these approaches will be elaborated below. 

Finally, in the context of these results, the well recognized, apparently lower thermal (and photochemical) stability of secondary cobalt-alkyls, relative to primary ones, may reflect the greater accessibility of irreversible decomposition pathways involving olefin elimination (i.e., through schemes such as that in Reactions 25-26), in addition to some probable lowering of the metal-alkyl bond-dissociation energy. 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Ng, F. T. T., Rempel, G. L., and Halpem, J., 1982, Ligand effects on transition-metal alkyl bond-dissociation energies, J. Am. Chem. Soc. 104 621n623. 

The stabilities of the metal-carbon bond formed from oxidative additions are as varied as their mechanistic pathways. Metal-carbon bond strengths increase going down a triad in an isostructural series of complexes. Alkyl migration to CO ligands on the metal to form acyl derivatives is more facile in first-row transition metals because of their lower metal-carbon bond energies. The thermal stability of alkyls vs. acyls does not follow any pattern, except that the availability of a sixth coordination site in ML (acyl) complexes favors the alkyl carbonyl isomer. The corresponding acyl, which can be made by running the reaction of the alkyl or aryl halide in CO (at 1-3 atm), is more stable by... 

After cis-addition of a monomer the geometry of an active center is reconstructed with a change of the positions of the vacancy and the alkyl. From the estimation of the contribution of the atomic orbitals to the molecular orbital localized at the Ti—C bond, it was concluded that the metal-alkyl bond is preserved during the reaction route. Cossee regarded the similar reactions to the concerted-type reactions which are characterized by synchronous redistribution of the electron density between dissociated and newly formed bonds which accounts for the low activation energies of these reactions. 

On the basis of their calculations Cossee and then Begley and Penella concluded that the activation energy of the insertion step depends on the energy AE of the transfer from the highest occupied orbital a, attributed to the metal-alkyl bond to the partly occupied or vacant d -orbital of the metal ion. However, this conclusion was not confirmed by or ab initio calculations. The... 

The observation of C-H, Si-H, Si-I, and Si-Si bond activation by Pd(0) and Pt(0) complexes has prompted theoretical studies of Sakaki and coworkers [29] concerning the C-H, C-C, and Si-X (X = H, C, Si, and F) bond activation on the Pt(PH3)2 and Pd(PH3)2 complexes. In these calculations the MP2 and MP4(SDQ) methods with the double-zeta quality basis sets argumented by the polarization d function and ECP for heavier atoms have been used. As shown in Table III, the oxidative addition of C-H and C-C bonds to Pt(PH3)2 is endothermic by 6.5 and 5.2 kcal/mol, respectively, while it is exothermic by 25.6, 28.6, 14.1, and 46.4 kcal/mol for Si-H, Si-F, Si-C, and Si-Si bonds, respectively. These differences in exother-micity of the reaction have been explained in terms of the Pt-X bond energies metal-alkyl bonds are a few kilocalories per mole weaker than the metal-H bond, which in turn is 0-15 kcal/mol weaker than metal-silyl or metal-F bonds. The activation barriers increase via the trend SiH (0.7 kcal/ mol) < SiSi (17.0 kcal/mol) < SiF (26.8 kcal/mol) < SiC (28.1 kcal/mol)... 

It is only during the past ten years that reliable and widely applicable methods for determining homolytic metal-alkyl bond dissociation energies of stable or-ganometallic compounds in solution have been developed and that information about such bond dissociation energies has become available. Today about one hundred transition metal-alkyl bond dissociation energies have been determined, the majority for cobalt-alkyl complexes. Most of these have been from kinetic measurements. The scope, limitations and results of such determinations are discussed. 

This paper is concerned with certain aspects of the thermodynamics and kinetics of transition metal-alkyl homolytic bond dissociation processes, notably of stable, ligated complexes in solution (L M-R, where L is a ligand and R = alkyl, benzyl, etc.)(l ). The metal-alkyl bond dissociation energy of such a complex (BDE, strictly bond dissociation enthalpy) is defined as the enthalpy of the process represented by Equation 1. 

In 1982 (23) we described a method of determining transition metal-alkyl bond dissociation energies, based on measuring the kinetics of reaction 1 and using the relation. 

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],... 

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