Metal carbides bond enthalpies

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. 

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... 

TABLE 4.3. Interatomic Distances (Angstrom) and Caicniated Bond Enthalpies (kcal moL ) of Some Metal Carbides... 

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

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