Bond enthalpy contributions bonding energetics

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

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. 

According to the definition of the A-B bond dissociation enthalpy, reactants and products in reaction 5.1 must be in the gas phase under standard conditions. That is to say that those species are in the ideal gas phase, implying that inter-molecular interactions do not exist. DH (A-B) refers, therefore, to the isolated molecule AB, and it does not contain any contribution from intermolecular forces. Though this is obviously the correct way of defining the energetics of any bond, there are many literature examples where bond dissociation enthalpies have been reported in solution. 

The exergonicity (AG°) is entirely enthalpy-driven, whilst a respectable negative entropy contribution testifies to the formation of a well ordered complex structure. The energetic signature is quite different from ordinary hydrophobic bonding in water which is commonly characterized by positive association entropies thereby reflecting the poor structural definition of the associate. 

The contribution of the Shockley surface state to the physisorption bond on Pt(lll) can even be quantified [45] on adsorption of 0.33 ML of Xe, the occupied Pt(lll) surface-resonance band at F is shifted from —0.40 to —0.25 eV because of hybridi2ation with the occupied Xe 5p level. This translates into an energetic cost of r 17 meV/Pt-surface-atom or 50 meV/Xe-atom. On Pd where the Shockley surface state is a priori unoccupied, an upshift of the surface state does not affect the total energy balance and no surface-state-related destabilization occurs. Hence, approximately 60% of the Xe adsorption enthalpy difference of 80meV/atom between Pt and Pd stem from the surface state ... 

---