Hydrogen atom, proton and electron transfer self-exc
hange and cross-reaction rates
have beendetermined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to t
heOs(IV) anilide TpOs(NHP
h)Cl
2 (
OsIVNHPh) gives t
he Os(III) aniline complex TpOs(NH
2P
h)Cl
2 (
OsIIINH2Ph)wit
h a new 66 kcal mol
-1 N-H bond. Concerted transfer of H
![](/images/entities/bull.gif)
between
OsIVNHPh and
OsIIINH2Ph isremarkably slow in MeCN-
d3, wit
h kexH![](/images/entities/bull.gif)
= (3 ± 2) × 10
-3 M
-1 s
-1 at 298 K. T
his
hydrogen atom transfer(HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to t
his HAT processare two proton transfer (PT) and two electron transfer (ET) self-exc
hange reactions, for instance, t
he ETreactions
OsIVNHPh +
OsIIINHPh- and
OsIVNH2Ph+ +
OsIIINH2Ph. All four of t
hese PT and ET reactionsare muc
h faster (
k = 10
3-10
5 M
-1 s
-1) t
han HAT self-exc
hange. T
his is t
he first system w
here all fiverelevant self-exc
hange rates related to an HAT or PCET reaction
have been measured. T
he slowness ofconcerted transfer of H
![](/images/entities/bull.gif)
between
OsIVNHPh and
OsIIINH2Ph is suggested to result not from a large intrinsicbarrier but rat
her from a large work term for formation of t
he precursor complex to H
![](/images/entities/bull.gif)
transfer and/or fromsignificantly nonadiabatic reaction dynamics. T
he energetics for precursor complex formation is related tot
he strengt
h of t
he
hydrogen bond between reactants. To probe t
his effect furt
her, HAT cross-reactions
have been performed wit
h sterically
hindered aniline/anilide complexes and nitroxyl radical species.Positioning steric bulk near t
he active site retards bot
h H
![](/images/entities/bull.gif)
and H
+ transfer. Net H
![](/images/entities/bull.gif)
transfer is catalyzed bytrace acids and bases in bot
h self-exc
hange and cross reactions, by stepwise mec
hanisms utilizing t
hefast ET and PT reactions.