Carbon bond energies

Table 2.3 Metal-carbon bond energies for some methyls...

Investigations of the cobalt-carbon bond energies in organometallic cobalt porphyrins continue to attract interest, originally because of their similarity to coenzyme B 2, and more recently because of their role in the catalysis of free radical... 

Interestingly as we have seen this may have a maximum as a function of the metal-carbon bond energy. 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

However, direct calculations of accurate bond energies represent a major challenge. Examples are given [13,14] where the ratios of carbon-carbon bond energies, relative to that of ethane, were successfully calculated for ethylene, acetylene, benzene, and... 

The problems to be solved are best illustrated by a typical example. Take sj, the carbon-carbon bond energy in ethane with Rqc = 1.531 A calculate the energy of a C(sp ) C(sp ) bond like that found in olefins. The latter is for reference charges, which are 35.1 me for the sp carbon (as in ethane) and 7.7 me for the sp carbon (as in ethylene). This transformation is schematically represented in Fig. 11.1. 

For example, the (ClgTPP)Fe- anion reacts with n-BuI, which provides a measure of the lower limit for the iron-carbon bond energy ... 

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. 

In aprotic solvents, direct electrochemical reduction of C02 (—2.23 V vs. SCE) yields carbon monoxide and carbonate ion.29 The (porT)Fe dianion also reduces C02 to CO, but at a less negative potential (-1.70 V).30 Hence, the estimated iron-carbon bond energy (—AGbF) for the (porT)Fera—C(0)0 dianion is at least 50 kJ mol-1 [—AGBF > 96.5(—1.70 + 2.23)]. 

Silicon-Carbon Bond Energy in Polydimethylsiloxanes and Alkyl-silanes. Technol. Rep. Osaka Univ. 10, 825—831 (1960) C. A. 55, 13959a (1961). 

Now a close approach is possible, and interaction of several orbitals can also occur. Besides the repulsive interaction between surface c bonds, a stabilizing interaction between fragment p orbitals also takes place. This results in a relatively low activation energy of recombination that only weakly depends on the metal-carbon bond strength. Competition between C-C chain growth and meth-anation or termination (CH formation) favours C-C chain growth as the metal-carbon bond energy increases. 

M. J. Wax, J. M. Stryker, J. M. Buchanan, C. A. Kovac, and R. G. Bergman, Reversible C—H Insertion/Reductive Elimination in ( 5-Pentamethylcyclopentadienyl)(trimethyl-phosphine)iridium Complexes. Use in Determining Relative Metal-Carbon Bond Energies and Thermally Activating Methane, J. Am. Chem. Soc. 106, 1121-1122 (1984). 

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... 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

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