Homolytic bond strength

The data necessary for thermodynamic estimates are available from experimental as well as computational methods. In many systems AGh can be approximated by experimentally accessible AGJ. The approximation is valid (to within 0.05-0.15 eV) if the radical coupling has no barrier (is diffusion limited) and the thermolysis is carried out under conditions selected to minimize the cage recombination [79]. The homolytic bond strengths can also be obtained in many cases from the Benson group-additivity tables [80] or semiempirical quantum or molecular mechanics calculations [81]. With appropriate entropy corrections [75f], relatively accurate AGh values can be obtained in that way. 

From the bonding picture in Scheme 5 and from perturbation theory, one can predict that the homolytic bond strength will be proportional to the orbital overlap and inversely proportional to the energy difference 1E,-E2 between the two participating orbitals. The prediction of the effect of a variation of oxidation state on the homolytic bond strength is not an easy excercise. The reason is that a change of oxidation state will independently affect E,-E2 and the orbital overlap. 

From the above studies, it is evident that the effect of an oxidation state change on the M-H homolytic bond strength is far from being clear, and results obtained on a specific system may not lend themselves to extrapolation or generalization. Further investigations on this deceptively simple chemical bonding topic are most definitely necessary. 

The relative M-X bond strengths in late metal amides and alkoxides have been measured by the studies on reversible M-X/H-X exchange (Equations 4.60 and 4.61). These essentially thermoneutral processes suggested a roughly 1 1 correlation of H-X and L M-X bond strengths, which provide an estimation of relative L,M-X homolytic bond strengths... 

Using these various approaches, homolytic bond strengths and self-exchange rate constants have been derived for a number of reagents. A selection of those used in this chapter are summarized in Table 1.1 " a more complete list of BDEs and BDFEs is given in references 14 and 39. 

The key characteristic of iimer-sphere reactions that can lead to high rates and selectivities is the strong covalent forces between the CH bond and M . The CH bond of methane is strong (homolytic bond strength of -105 kcal/mol) and an important feature of the CH activation reaction is the formation of strong M-C bonds that compensate for breaking the CH bond. This is one of... 

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