Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. Fe-II(H(2)bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and Fe-II(H(2)bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H-center dot to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, Fe-III(Hbip) or Fe-III(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For Fe-II(H(2)bip) + TEMPO, Delta G degrees = -0.3 +/- 0.2 kcal mol(-1), Delta H degrees = -9.4 +/- 0.6 kcal mol(-1), and Delta S degrees = -30 +/- 2 cal mol(-1) K-1. For Fe-II(H(2)bim) + TEMPO, Delta G degrees = 5.0 +/- 0.2 kcal mol(-1), Delta H degrees = -4.1 +/- 0.9 kcal mol(-1), and Delta S degrees = -30 +/- 3 cal mol(-1) K-1. The large entropy changes for these reactions, parallel to T Delta S degrees parallel to = 9 kcal mol(-1) at 298 K, are exceptions to the traditional assumption that Delta S degrees approximate to 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed Delta S degrees(HAT). Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, parallel to Delta S degrees(Fe(H2bim)/ET)parallel to = 29 +/- 3 cal mol(-1) K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [Fe-II(H2NCHCHNH2)(2)(H(2)bim)](2+)center dot center dot center dot ONH2 -> [Fe-II(H2NCHCHNH2)(2)(Hbim)](2+)center dot center dot center dot HONH2. The discovery that Delta S degrees(HAT) can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.