H Transfer Between Ligands and Organic Radicals

The transfer of H to organic radicals is not limited to hydride complexes. Although free N-H and O-H bonds have high bond dissociation enthalpies, they are weak- 

Mayer has observed transfer of H to TEMPO from an N-H bond in the tris iron(II) complex of 2,2 -bi(tetrahydropyrimidine) (1.11), and has shown that the Marcus cross relation accurately models its negative enthalpy of activation [38]. As previously suggested in another context [39], the high point on the enthalpy surface appears to occur before the transition state. 

The H transfer in Equation 1.13, to a vanadium oxo ligand, is surprisingly slow. The transfer requires a great deal of structural reorganization, as the V-O distance increases by 0.264A when V=0 is converted to V-OH. The barrier to transfer of an H from V-OH to V=0 (in Marcus theory, the intrinsic barrier to selfexchange) is thus large, and its size is reflected in the size of the barrier to the C— 0 H transfer in Equation 1.13 [46], 

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