environmental relevance, because only the photoredox behavior is
able to induce oxidative photodegradation of the pollutants.
To undergo a photoredox reaction, the central atom should
coordinate an electron donor good enough for electron transfer
toward the Cr(III) center in the LMCT excited state. Photoredox
reactivity was reported for Cr(III) complexes with ligands such
as N 3
( 247 ), CN
( 248 ), NCS
( 249 ), edta (18,244), and C 2 O 42

( 245 ). The LMCT excitation within the UV range was shown to
induce Cr(III) photoreduction to Cr(II) in [Cr(CN) 6
x(OH)x]^3

( 248 ), [CrA 2 (NCS) 4 ]
(A¼NH 3 , aniline (C 6 H 5 NH 2 ), n-
propylamine (C 3 H 7 NH 2 ), morpholine (NHCH 2 CH 2 OCH 2 CH 2 )),

and [Cr(A

A)(NCS) 4 ]
(A–A¼1,2-diaminethane or 1,2-
diaminopropane) complexes ( 250 ). The Cr(III) complexes with
edta are able to undergo photoreduction upon excitation by solar
light (18,244), whereas the photoreactivity of oxalate and citrate
complexes needs some more energetic radiation (245,251,252).
The LMCT excitation of the Cr(III) complex with edta:

CrIIIðÞedtaOH

hi 2 

!

hvðÞLMCT

* CrIIðÞedtaOH

hi 2 


ð 48 Þ

is followed by back electron transfer

 CrIIðÞedtaOH

hi 2 


! CrIIIðÞedtaOH

hi 2 


ð 49 Þ

and/or PET accompanied by dissociation of the oxidized part of
the ligand. In the presence of molecular oxygen, *[CrII(edta)
OH]^2
is quenched reproducing Cr(III) or even yielding Cr(VI)
species. The latter Cr(II) oxidation (Eq. 50) is possible under spe-
cific conditions, that is, in alkaline medium and at a relatively
large excess of O 2 over the Cr(II) concentration (18,244,248– 250 ).

Cr IIðÞaqþO 2!

OH


CrO 42 
 ð 50 Þ

Another hypothesis concerning the mechanism of the Cr(II)!Cr
(VI) oxidation is based on the assumption that in alkaline
medium the edta^3
radical ligand, similar to the free edds^3

radical (Eq. 37), produces OHradicals (225,252).
The crucial role of OHradicals in Cr(II)!Cr(VI) oxidation
was suggested also in the case of UV irradiated citrate complexes
[CrIII(cit)] and [CrIII(cit)OH]
( 252 ). The reaction series is
initiated by photoreduction:

CrIIIðÞcit

hi

!

hvðÞLMCT

 CrIIðÞcit

hi

!CrIIaqþcit ð 51 Þ

326 ZOFIA STASICKA