Hydrogen atom transfer (HAT) reactions of the bis(histidine) cytochrome active site models (TPP)Fe-II(ImH)(2) (Fe(II)ImH) and (TPP)Fe(Im)(ImH) (Fe(III)Im) have been examined in acetonitrile solvent (TPP = tetraphenylparphyrin, ImH = 4-methylimidazole). The ascorbate derivative 5,6-isopropylidine ascorbate, hydroquinone, and the hydroxylamine TEMPOH all rapidly add H-center dot to Fe(III)Im to give Fe(II)ImH. Similarly, the phenoxyl radical 2,4,6-(Bu3C6H2O center dot)-Bu-t and excess TEMPO center dot each oxidize Fe(II)ImH to give Fe(III)Im. On the basis of redox potential, pK(a), and equilibrium measurements, the N-H bond in Fe(II)ImH was found to have a bond dissociation free energy (BDFE) of 70 +/- 2 kcal mol(-1). A hydrogen atom transfer mechanism (concerted transfer of e(-) and H+) is indicated based on data for the ascorbate and TEMPO center dot reactions.