Precursor and Successor Complexes for HAT

The Marcus theory model is derived for unimolecular electron transfer. It is applied to bimolecular reactions by assuming that the reactants weakly associate in a precursor complex within which ET occurs to give the successor complex. The cross relation analyses above have implicitly adopted this same model, but HAT precursor complexes are quite different then ET ones. This is because proton transfer occurs only over very short distances, so HAT precursor complexes have distinct conformations, rather than the weakly interacting encounter complexes of ET. In this way, HAT resembles proton transfer and inner-sphere electron transfer. Including the equilibria for precursor and successor complex formation expands equation (1.1) into equation (1.20). 

One case where the importance of HAT precursor and successor complexes is clear is the reaction of [Co(Hbim)(H2bim)2] (abbreviated to Co(Hbim)) with TEMPOH (eqn (1.21)). Reaction (1.22) shows kinetic saturation behavior in the forward direction, indicating pre-equilibrium formation of a Co(Hbim) TEMPOH precursor complex with Kp = 61.3 0.8 M (A(jp° = -2.44 0.05 kcal moP ). In the reverse direction, saturation was not observed but kinetic models indicate AGs° = 0.24 0.83 kcal mol . Taking 

A more complete description of HAT reactions would also include the presence of hydrogen-bonded solvent molecules, which (as described in Section 1.3 above) must dissociate prior to formation of the precursor complex (eqn (1.22) formation of the successor complex is similar). A detailed model including these equilibria is presented in reference 14 and its Supporting Information. 

Summary of observed and calculated (eqn (1.7)) rate constants for transition metal PCET reactions.  

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