Stepwise and Mean Bond Dissociation Enthalpies

Experimental values of bond dissociation enthalpies are scarce compared with the data available for standard enthalpies of formation. This is not surprising because most chemical reactions that have been studied thermochemically involve the cleavage and the formation of several bonds. The measured standard reaction enthalpies are thus enthalpy balances of various bond dissociation enthalpies, whose individual values are often unknown. Consider, for example, reaction 5.10, where the arene ring in (q6-bcnzene)chromium tricarbonyl is replaced by three carbonyl ligands. The enthalpy of this reaction at 298.15 K, 

To derive the chromium-benzene bond dissociation enthalpy, the values of several stepwise bond dissociation enthalpies in chromium hexacarbonyl are 

Now suppose that no data were available for the stepwise Cr-CO bond dissociation enthalpies and we wanted to estimate DIt° (CO)3Cr CV, IU ]. A simple approach starts by considering reaction 5.15, where all the Cr-CO bonds are cleaved. 

Although we ignore the values for some of the six stepwise Cr-CO bond dissociation enthalpies in chromium hexacarbonyl, their sum is equal (equation 5.16) to the enthalpy of reaction 5.15, which is calculated as 641.7 4.8 kJ mol-1 (at 298.15 K) from the well-known standard enthalpies of formation of all the species involved [16,17,31], 

If 641.7 kJ mol-1 is required to break the six Cr-CO bonds in Cr(CO)e, we may consider that on average, the cleavage of one bond corresponds to (641.7 4.8)/6 = 107.0 0.8 kJ mol-1. Therefore, breaking three bonds will require about 321 2 kJ mol-1. If we now assume that this value applies to the cleavage of the three Cr-CO bonds in Cr(CO)3(C6H6), we obtain DII° (CO Cr CeHe] = 177 11 kJ mol-1. Unfortunately, this result is some 72 kJ mol-1 lower than the best value mentioned, 249 43 kJ mol-1. 

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