FeIIIðÞedtaðÞH 2 O

hi

!

hnðÞLMCT 

FeIIðÞedtaðÞH 2 O

hi


ð 29 Þ

with the edta^3
radical coordinated to the FeIIcenter by means
of five COO
groups with a radical CH 2 COOgrouping pending
free in the transient species. The simultaneous presence of a
vacant coordination site and radical ligand in a complex anion
is responsible for quenching the transient by both electron
donors and acceptors, or even for self-quenching(16,17):

 FeIIðÞedta H

ðÞ 2 O

hi


þ FeIIIðÞedtaðÞH 2 O

hi


!

ðÞH 2 OðÞedtaFeIIðÞm
OHxFeIIIðÞedta

hix
 4 ð^30 Þ

Within milliseconds the Fe(II)/Fe(III) intermediate undergoes
inner-sphere electron transfer:

ðÞH 2 OðÞedtaFeIIðÞm
OHxFeIIIðÞedta

hix
 4

þxH 2 O!

ðÞedtaFeIIIðÞm
OFeIIIðÞedta

hi 4 


þxH 3 Oþ

ð 31 Þ

and regenerates the original Fe(III) complex:

½ŠðÞedtaFeðÞm
OFe edtaðÞ
4

þ2H 3 Oþ
!2 Fe

IIIðÞedta H

ðÞ 2 O

hi


þH 2 O

ð 32 Þ

Quenching *[FeII(edta)(H 2 O)]
by electron donors leads in a
deoxygenated medium to the regeneration of the edta^4
ligand
from the edta^3
radical and formation of the reduced Fe(II) com-
plex and oxidized donor molecule (D¼aliphatic alcohol, sulfate
(IV), nitrate(III) or even extraneous edta^4
ligand):

FeIIðÞedta H

ðÞ 2 O

hi


þD! FeIIðÞedta H

ðÞ 2 O

hi 2 


þDþ ð 33 Þ

whereas in the presence of O 2 or another external electron acceptor
(e.g., A¼CrO 42
), the excited state undergoes oxidative quenching:

 FeIIðÞedta H

ðÞ 2 O

hi


þA! FeIIIðÞed3a H

ðÞ 2 O

hi

þA
þedtaox

ð 34 Þ

producing a stable Fe(III) complex with pentadentate
ethylenediaminetriacetate ligand ((OOCCH 2 ) 2 NCH 2 CH 2 NH
(CH 2 COO)^3
, ed3a), and oxidation products of one part of the
edta^4
ligand (CH 2 COOradical) ( 6 ).

320 ZOFIA STASICKA