Bond enthalpy contributions transferability

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. 

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) ... 

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