Metal-carbon bond dissociation enthalpies

Thermodynamic aspects of the insertion reactions have been considered, especially with regard to reactions (b), Y = O. The enthalpy change of the reaction is related to the disruption of the metal-carbon bond, the formation of the new metal-carbon bond, the formation of the new carbon-carbon bond, and the reduction of the CO bond order. From the estimated metal-carbon bond dissociation enthalpies (BDE) of reagent and product and from the energetics of the other quantities involved in the process, the AC of reaction (g) has been calculated to be negative for R = Me or Ph and positive for R = CF3. This is in agreement with the observation that the trifluoroacetyl derivative... 

To ensure thermochemicai consistency, the H-H and C-H bond dissociation enthaipies were aiso quoted from Ref 124. In the cases of ethane, butane, benzene, and toluene, these are not the most updated values (see, for instance, Blanksby, S. J. Ellison, G. B. Acc. Chem. Res. 2003, 36, 255), but the (upward) adjustments are smaller than the uncertainties of the metal-carbon bond dissociation enthalpies. 

Table 1.1. Mean metal—carbon bond dissociation enthalpies D(M—Me)/kJmol for compounds Me M. ... 

Metal—Carbon and Metal—Hydrogen Bond Dissociation Enthalpies from Classical and Nonclassical Calorimetric Studies... 

This implies that the reaction will be favorable (A,G (60)<0) only if A, (60) = Z) (Me-H)— DIP (M-H) - DIP (M-Me) < —44 kJ mol, that is, the sum of metal—carbon and metal—hydrogen bond dissociation enthalpies must exceed the carbon-hydrogen bond dissociation enthalpy in methane, 439 kJ moF , by that amount. Taking a middle range value for Z)//°(M-H), 250kJmoF (Table 1), one concludes that Z) (M-Me) should be higher than ca. 230kJ moF. ... 

Bond dissociation enthalpies (BDEs see Box l.l) for transition metal o-organyls appear at present to fall within the quite wide range of 70-330 kJ mol-1, i.e. a range comparable to those of main group alkyls. It should be noted, however, that transition metals also have the possibility of including a 7t-component when the carbon is sp2 (alkylidenes, alkenyls, acyls) or sp hybridized (alkylidynes, vinylidenes, alkynyls), thereby leading to larger BDEs. 

Table 15.7 Bond dissociation enthalpies, D, for metal-carbon bonds0...

Caleulations have indicated that bonds between transition metals and terminal carbon atoms are quite strong, with bond dissociation enthalpies comparable to those of... 

Barriers to rotation around the Cca —N bonds have been determined experimentally for diaminocarbenes (3) and (4) and their protonated and lithiated counterparts the possible involvement of lithium or a proton in the dimerization of these acyclic diaminocarbenes was also reported. A computational study of the dimerization of diaminocarbenes has been performed via rate constant calculations using general transition-state theory calculations. Such a dimerization has been shown to be a rapid equilibrium between the carbenes and the tetra-A-alkyl-substituted enetetramines (5), by characterization of metathesis products when two different tetramines were mixed. The thermodynamic parameters of this Wanzlick equilibrium have been determined for the A-ethyl-substituted compound the enthalpy of dissociation has been evaluated at 13.7kcalmol and the entropy at 30.4calmor K . Complex-ation of diaminocarbenes by alkali metals has been clearly established by a shift of the C NMR signal from the carbene carbon of more than 5 ppm. ... 

---