The transfer of a hydrogen atom-a proton and an electron-is a fundamental process in chemistry and biology. A variety of hydrogen atom transfer reactions, involving iron complexes, phenols, hydroxylamines, tBuOOH, toluene, and related radicals, are shown to follow the Marcus cross relation. Thus, the Marcus theory formalism based on ground-state energetics and self-exchange rates, originally developed for electron transfer processes, is also valuable for hydrogen atom transfer. Compounds that undergo slow proton transfer (C-H bonds) or slow electron transfer (cobalt complexes) also undergo slow hydrogen atom transfer. Limitations of this approach are also discussed.

Described here are oxidations of alkylaromatic compounds by dimanganese mu-oxo and mu-hydroxo dimers [(phen)(2)Mn(IV)(mu-O)(2)Mn(IV)(phen)(2)](4+) ([Mn(2)(O)(2)](4+)), [(phen)(2)Mn(IV)(mu-O)(2)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(2)](3+)), and [(phen)(2)Mn(III)(mu-O)(mu-OH)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn(2)(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn(III)(mu-OH)(2)Mn(II)(phen)(2)](3+) ([Mn(2)(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn(2)(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+). [Mn(2)(O)(2)](4+) is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn(2)(O)(2)](3+). The oxidation of toluene by [Mn(2)(O)(2)](4+), however, is not inhibited by [Mn(2)(O)(2)](3+). Oxidation of a mixture of C(6)H(5)CH(3) and C(6)H(5)CD(3) shows a kinetic isotope effect of 4.3 +/- 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn(2)(O)(2)](4+). Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e(-), H(*), and H(-) transfers have been determined from redox potential and pK(a) measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-)(1) affinities for H(*). In the oxidation of p-methoxytoluene by [Mn(2)(O)(2)](4+), hydride transfer is thermochemically 24 kcal mol(-)(1) more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.

Formation constants have been determined for the complexes MoO(O-O)(dipic) and MoO(0-0)(nta)" at 25 °C with / = 0.10 M (NaCl). For the general formula ( 4)( )( +)4( 2 2), log K¡ values of 23.48 and 23.0 were determined for L = dipic by potentiometry and spectrophotometry, respectively; for L = nta, the corresponding log Rvalues were 27.87 and 28.8. A single-crystal X-ray structure was determined for the nta complex, and 95Mo NMR spectra were collected. The dipic complex is much more labile than the nta analogue. The rate of loss of peroxide from the complexes was measured by use of the peroxide "trapping agents" S(IV) and Zr(IV); first-order rate constants (s-1) at 25 °C were, for dipic, 0.03 at pH 1 and, for nta, 1 X 10-4 at pH 1 and 5 X 10"6 at pH 4. Rate constants (M™1 s-1, at 25 °C) for the oxygen atom transfer reactions with (en)2Co(SCH2CH2NH2)2+ and (CH3)2SO were, for MoO(0-0)(dipic), 8.6 X 10* 12and7.6X 10~3and,forMoO(O-O)(nta)~,2.5 X 103 56and5.8X 10"3, respectively. These reactivities are approximately the same as observed previously with the oxo diperoxo complex MoO(OH)-(0-0)2". Activation of coordinate peroxide was also observed in the kinetics of reduction by iron(II), with rate constants of 260 and 3.0 X 103 M-1 s~' at 25 °C for the dipic and nta oxo monoperoxo complexes, respectively. No intermediate was detected in either reaction, whereas one formulated as a superoxo complex is formed in the reduction of oxodiperoxomolybdates by iron(II).

The reaction of a 1:1 mixture of (H(2)O)(5)Cr((16)O(2))(2+) and (H(2)O)(5)Cr((18)O(2))(2+) at pH 1 did not yield measurable amounts of (16)O(18)O. This result rules out a Russell-type mechanism (2(H(2)O)(5)CrO(2)(2+) --> 2(H(2)O)(5)CrO(2+) + O(2)) for the bimolecular decomposition reaction. Evidence is presented in support of unimolecular (S(H)1) and bimolecular (S(H)2) homolyses as initial steps in the decomposition of (H(2)O)(5)CrO(2)(2+) in strongly acidic solutions (pH