Transition Metal Systems that Deviate from the Cross Relation

5 Transition Metal Systems that Deviate from the Cross Relation 

The origin of the deviation from the cross relation for the reactions of [V (0)2(bpy)2] or [Ru (0)(bpy)2(py)] with xanthene is not known. Three other HAT reactions of xanthene do not show this large deviation (Table 1.4), so the deviations are not likely to be due to an erroneous xanthene selfexchange rate constant [taken as that of 9,10-dihydroanthracene (DHA, Table 1.1)]. Tunneling could be important in reactions of Ru (0)(bpy)2(py), as the reaction with DHA has 35. Interestingly, the cross relation 

5 Conclusions Implications and Limitations of the Cross Relation for Hydrogen Atom Transfer Reactions 

The success of this model is notable for a number of reasons. In particular, it is remarkable that the model holds so well for such a wide variety of reactions and reactants. Linear free energy relationships (LFERs) relating rate constants with driving force e.g., Bronsted relationships) are a very useful part of reaction chemistry, but they are essentially always limited to a set of closely related compounds and reactions. LFERs such as AG — aAG° + P have parameters (a,j8) that are defined only by this relationship. In contrast, the values that enter into the cross relation, xh/y xh/x and A yh/y and the parameters for the KSE model (a2 and 2 ), are all independently measured and have independent meaning. There are no adjustable or jittedparameters in this model. 

This general agreement with the cross relation appears to hold only for HAT and for outer-sphere electron transfer. The other examples of application of the cross relation, to transfer, H transfer, and Sn2 reactions, only hold within a limited subset of similar reactions.The critical feature here appears to be the broad applicability of the additivity postulate for HAT, that the intrinsic barrier for a reaction is well estimated by the average of the 

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