Carbon bond strengths

As an example of carbon bond strengths the data for tire stages in the atomization of methane are shown below, together for tire less accurate data for silane... 

J. A. Martinho Simoes, J. L. Beauchamp. Transition Metal-Hydrogen and Metal-Carbon Bond Strengths The Keys to Catalysis. Chem. Rev. 1990, 90, 629-688. 

The nature of the central metal ion. In principle the redox potential of the Mre+1/re couple and the stabilization of the M" + 1 ion by the ligand R affect the metal-carbon bond strength. The stability of the transient complex is also affected by the rate of ligand exchange of the LmM"+1 R complex. 

The Raman spectrum of the dimethylaurate(I) anion indicated a linear structure (2/9), and the bands were found to correlate with those of Me2Hg, Me2Tl+, and Me2Pb2+. The metal-carbon bond strength decreased in the order Au > Hg > T1 > Pb, opposite the expected trend in terms of nuclear charge. 

Boron-oxygen bond strengths (480-565 kJ mol 1) are greater than boron-carbon bond strengths (350-400 kJ mol 1). This reflects an interaction between the empty p orbital on boron and an electron pair in one of the oxygen s two filled non-bonding sp3 orbitals. 

Relativistic effects on the metal-carbon bond strengths of Me2M, where M = Au-, Hg, T1+ and Pb2+, were studied by Schwerdtfeger183. The author found that in Me2Pb2+ the BDEs of the Pb2+ —Me bonds increase by ca 15% and the force constant of the symmetric Me—Pb2+—Me stretching mode by ca 20% when relativistic effects are included in the calculations. 

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. 

Metal hydrogen and metal - carbon bond strengths, inclnd-ing some values for zinc, have been reviewed. ... 

Table 7.2 Carbon-carbon bond strengths and lengths in fluoroethanes [3]...

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... 

The thermal reactions of the pyridinium borate salts are likely to follow the same electron-transfer path. Experimental evidence for this conclusion is the fact that the 5cc-butyl transfer is substantially faster than methyl transfer although a nucleophilic substitution mechanism would predict the less hindered group to be transferred preferentially. The fast rates of 5cc-butyl transfer can be readily explained on the basis of the electron-transfer mechanism (Eqs. 69-71) by considering the different boron-carbon bond strength [189, 190] for the various alkylborates. The boron-carbon bond cleavage (Eq. 70) is apparently the critical step, and its relative rate [191] as compared to that of the back electron transfer determines the overall rate for thermal alkyl transfers in pyridinium tetraalkylborate salts. 

In principle, the above selectivity problems can be avoided in suitably designed homogeneous metal-ion-catalyzed oxidation procedures. Transition metals, particularly those whose most stable oxidation states differ by 2e", often promote nonradical pathways even in the presence of dioxygen [6]. Moreover, since metal-carbon bond strengths parallel those of C-H bonds and because of steric factors, the preferential functionalization of primary C-H bonds becomes possible [7]. As a bonus, metal-ion catalyzed reactions usually operate at low temperatures (-100 °C or below) [8]. 

Double insertions of carbon monoxide into the same metal-hydrocarbyl bond [reaction (j)], are doubtful and multiple insertions [reaction (k)] are unknown. This should be attributed to the relatively lower strength of the carbon-carbon bond in a sequence of the type —C(0)—C(0)—, as shown by the available bond energy data . For example, while the carbon-carbon bond strength in ethane is 376 kJ/mol, the corresponding value in MeC(O)—C(0)Me has been evaluated to be 282 kJ/mol... 

The higher metal-carbon bond strength for a third-row metal explains the failure of rhenium to react. Carbon monoxide insertion occurs in the -cyclopentadienyl derivative of rhenium(I), Re(NO) (>7 -C5H5)Me(PMe3) ... 

Another system for which thermodynamic data have been obtained in some detail is the Tp Rh(CNneopentyl)(R)H system studied by Jones. Here, the relative thermodynamic stabilities of a number of adducts were obtained by measuring both the competitive kinetic selectivity for two types of C-H bond (AAGt in Fig. 2) as well as the barrier for reductive elimination of free alkane from each adduct (AG and AG in Fig. 2). The free energies for the latter were obtained from kinetic studies of the reductive elimination of hydrocarbon in benzene. A summary of the AG° values, calculated equilibrium constants, and relative metal-carbon bond strengths are given in Table 4 [26]. For DC H for benzene, see ref. 

Wolczanski also investigated the chemistry of a tantalum imido system. In this system, elimination of hydrocarbon from the bis-amido imido complex occurs with difficulty at 183°C to give an amido bis-imido complex. The elimination is reversible, with the bis-imido species not being directly observed (Scheme 10). Under methane pressure, the phenyl complex loses benzene and adds methane. Neopentane, benzene, and toluene (benzylic activation) were also found to undergo activation, but not cyclohexane. The authors conclude from their equilibrium studies that the differences in metal-carbon bond strengths are approximately equal to the differences in carbon-hydrogen bond... 

The shearing process itself is a simple homolytic scission of a carbon-carbon bond to produce two polymeric radicals. There is no evidence that these radicals are involved in other aspects of lubricant chemistry and are apparently rapidly terminated by either antioxidant or other species in the oil. Backbone carbon-carbon bond strengths of all of the VI improver are much the same so none of the chemistries is inherently more stable than any other. Several workers have proposed that at least some of the shearing takes place at weak links, such as might result from steric hindrance, but there is little or no evidence to support this hypothesis. 

---