Transferability, bond enthalpies

When there are bonds of two or more kinds in a molecule, the determination of the bond enthalpies is slightly more complicated and is based on the assumption that bond enthalpies can be transferred from one molecule to another, at least to a reasonable approximation. For example, the enthalpy of atomization of ethane, which is 2826 kJ mol-1, is the sum of six C—H bond enthalpies and one C—C bond enthalpy. The C—C bond enthalpy can be determined only if we make an assumption about the C—H bond enthalpy. If we make the rea-... 

The principal assumption of transferability (Section 1.3.) seems to be acceptable on the basis of the data accumulated so far, but clearly requires further testing where organic ligands are concerned. No alternative assumption suggests itself in relation to M-CO bond enthalpies, but the resulting values of T and M could be improved when measurements are made on the more exotic polynuclear metal carbonyls that have been prepared in the last few years. 

Initial DFT calculations suggested the assignment of 28 as having Al+ with a primarily cr-donation of the lone pair from A1 to Fe. From the DFT calculations, the Al-Fe bond enthalpy was calculated to be 52.7 kcal mol-1, a value that was considered to be overestimated, and a better value was thought to be closer to 50 kcal mol-1.39 This view was revised by later work that indicated that there was a large transfer of electron density from A1 to Fe, so that an RA12+ bound to Fe(CO)42- was deemed more appropriate.41... 

We have shown that the result of replacing stepwise bond dissociation enthalpies by mean bond dissociation enthalpies and transferring bond dissociation enthalpies from one molecule to another can be deceptive The assumption that in Cr(CO)3 (C6H6), DH° (Cr-CO) + DH° Cr-CO) + DH° Cr-CO) 3 (DH°)( Cr CO) led to an error of 72 kJ mol-1 in DH°[(CObCr-CeHe]. Yet this error cancels out if the same procedure is applied to derive relative Cr-arene bond dissociation enthalpies in a series of ( r 6-arcnc)chromium tricarbonyl complexes. 

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. 

In summary, the previous example shows that bond dissociation enthalpies should not be correlated with bond lengths unless the relaxation energies of the fragments are comparable. On the other hand, when two bonds between the same pairs of atoms have identical bond lengths, it is sensible to assume that they have similar bond enthalpy contributions. Hence, in this case, a bond enthalpy contribution can be transferred from one molecule to another. 

Concomitant with these developments in spectroscopy, thermochemists were finding that, to a reasonable approximation, molecular enthalpies could be determined as a sum of bond enthalpies. Thus, assuming transferability, if two different molecules were to be composed of identical bonds (i.e., they were to be isomers of one kind or another), the sum of the differences in the strains of diose bonds from one molecule to the other (which would arise from different bond lengths in the two molecules - the definition of strain in this instance is the positive deviation from the zero of energy) would allow one to predict the difference in enthalpies. Such prediction was a major goal of the emerging area of organic conformational analysis. 

Is there a corresponding decomposition for nonelectrolytes One might suppose that the analogous building blocks of a nonelectrolyte compound are its chemical bonds. Based on the known similarity and transferability of particular bond types from one molecule to another, one could then attempt to assign each bond a specific bond enthalpy (denoted 7)//°[bond]) such that the overall sum of DH°s is related to (the negative of) AHf by... 

For molecules M2(NMe2)g, AHp = 6D(M-NMe2) + D(M M) the nature of the problem is immediately apparent. What value of D(M NMe2) should be transferred into the equation to derive D(M-M) Any error in D(M-NMe2) enters sixfold into the derived value. For example, the formal oxidation-number of the metal, M(III) in M2(NMe2)6 yields values of D(Mo -Mo) 200, and D(W-W) = 340 kJ mol-1 which are close to the respective metal-ligand bond enthalpy contributions. On the other hand, each metal atom has a valency of six, which yields values of D(HPM) (Mo, (788 24) ... 

As far as equation (55a) is concerned, the enthalpy term is accurately known for most metals, corresponding to the transfer of the solid metal to its state of a monoatomic gas in its high-spin ground-state configuration. Equation (55b) contains the negative value of the bond enthalpy, which, divided by the number of M O bonds in the molecule, gives the mean bond dissociation enthalpy. M(CO) (g) differs from M(CO) (s) or M(CO) (l) by the heats of sublimation or vaporization, respectively. 

Bond enthalpy terms, on the other hand, are quantities assigned to each bond in a molecule such that the sum over all bonds is equal to the enthalpy change associated with the conversion of the molecule into separate atoms. Bond enthalpy terms are assumed to be constant, and therefore transferable from molecule to molecule. So for methane ... 

Mean bond enthalpies are defined as the enthalpy changes involved in breaking bonds in molecules. They may be determined from thermochemical cycles using Hess Law, although assumptions of transferability are sometimes required. 

OH has changed from the normal situation where oxygen forms two bonds. The assumption of transferability involved in this method of determining bond enthalpies is, however, open to question (see below). 

Tabulated values of bond enthalpies can be used to estimate the enthalpy of formation of hypothetical compounds. Such estimates should be regarded as rough and not quantitatively reliable, as the assumptions of additivity and transfer ability that underlie these calculations are not accurate. 

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