ligand like pyridinedicarboxylate (pda). This behavior was
interpreted by the reaction series:
CrIIIðÞC 2 O 42 ðÞpdahi 3 !
hnðÞLMCT
CrIIðÞC 2 O 4 ðÞC 2 O 4 ðÞpdahi 3 ð 61 ÞCrIIðÞC 2 O 4 ðÞC 2 O 4 ðÞpdahi 3
! CrIIIðÞC 2 O 4 ðÞC 2 O 4 pda^3
hi 3 ð 62 ÞCrIIIðÞC 2 O 4 ðÞC 2 O 4 pda^3 hi 3
!CrIIIðÞC 2 O 4 ðÞC 2 O 4 ðÞpdahi 2
þeaqð 63 ÞCrIIIðÞC 2 O 4 ðÞC 2 O 4 ðÞpdahi 2
þ2H 2 O!CrIIIðÞC 2 O 4 ðÞpdaðÞH 2 O 2hi
þC 2 O 4 ð 64 Þdemonstrating that photoinduced innersphere charge transfer
from the oxalate ligand to the Cr(III) center (Eq. 61) was followed
by secondary inner-sphere CT from the Cr(II) center to the
pyridinedicarboxylate ligand (Eq. 62) and by outer-sphere elec-
tron transfer to the solvent (Eq. 63).
Molecular oxygen competes effectively with inner- or out-
ersphere electron transfer, and production of chromate(VI) was
recorded both in the case of oxygenated homo- and heteroleptic
complexes with the quantum yield decreasing within the series
2,3-pda>2,4-pda>2,5-pda(ox) 3. However, Cr(VI) production
was never recorded in the photoreaction of homoleptic complexes
containing only pyridinedicarboxylate ligands( 251 ).
The photoredox reaction of chromium(III) complexes with lig-
ands such as oxalate, citrate, or edta can proceed under environ-
mental conditions. In consequence, these pollutants undergo
oxidative degradation, but simultaneously the harmful and
toxic chromate(VI) is generated. To close the photocatalytic cycle,
Cr(VI) had to undergo successive photoredox processes.
C.2. Chromate(VI) compounds
In the environment, chromate(VI) occurs in two forms,
depending on the solution pH (pK¼6.5):
HCrO 4 !CrO 42 þHþ ð 65 Þ328 ZOFIA STASICKA