Metal-carbon bond enthalpies, calculated

Because the enthalpies of formation of such molecular metal carbonyl carbide clusters have yet to be measured or calculated accurately, it has not been possible to calculate their metal-carbon bond enthalpies. However, enough thermochemical information is available on some bulk metal carbides to allow the strengths of both their metal-metal and metal-carbon bonds to be assessed, as indicated in the next section. 

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... 

Bond enthalpies thus calculated for early transition metal carbides MC and M2C are given in Table 4.3. The carbon atoms occupy octahedral holes in the metal lattice in all cases except WC, in which the carbon sites are trigonal... 

The equation generating bond enthalpies / (Re-Re) (kJmoR ) from bond distances d (pm) that fits the known enthalpy of vaporization of rhenium metal (770 kJ mol ) is E = 2.188 x 10 By use of this equation we have calculated Eii(Re-Re) for all the systems that have been structurally characterized, and in Fig. 8 we plot (Re-Rej/x against the number of electrons per metal atom provided by the metal (v = 7 for Re) plus the ligands, core carbon and/or anionic charge (xv + 2y + z + c + i)/x, where i is the number of electrons supplied by the interstitial atom, four for carbon. 

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. ... 

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