inorganic chemistry

complexes are initiated by CT excitation, which in simple cases
leads to a one-electron transfer between metal and ligand or
another substrate (2,3). Accordingly, radicals are generated that
are very reactive. They undergo various secondary reactions
including recombinations. In principle, such one-electron
photoredox reactions can be combined to produce relatively stable
products that are formally the result of a multielectron transfer.
However, to put this in reality is extremely difficult. Fortunately,
such interferences may be avoided by the introduction of suitable
metal complexes that undergo multielectron transfer ( 4 ).
There are essentially two possibilities to accomplish two-elec-
tron or multielectron transfer at metal complexes without forma-
tion of one-electron transfer intermediates (e.g., radicals).
Appropriate metal centers should have available stable oxidation
states that differ by at least two units. Typical examples that
represent such photoredox reactions are reductive eliminations
such as (X
¼halide, pseudohalide)( 5 ):

PtIVðÞCN 4 X 2

hi 2 


!

hn

PtIIðÞCN 4

hi 2 


þX 2 ð 5 Þ

LMCT (X
to PtIV) excitation is associated with a shift of electron
density from X
to PtIVbut finally yields Pt(II) because Pt(III) is
not easily accessible. In cases that are restricted to one-electron
transfer at single metal centers a simultaneous multielectron
transfer can be achieved by assembling two or more metal
centers in binuclear or polynuclear complexes( 4 ). This applies,
for example, to the redox couple Fe(II) and Fe(III)
(PctsH 2 ¼phthalocyaninetetrasulfonate) ( 6 ):

ðÞPctsFeIII O 22 



FeIIIð Þ!Pcts

hn

2FeIIð ÞþPcts O 2 ð 6 Þ

LMCT (O 22
to FeIII) excitation finally leads to the formation of
Fe(II) and molecular oxygen.
In conclusion, photochemical multielectron transfer of metal
complexes has been frequently studied with the intension to real-
ize artificial photosynthesis. In the following sections, this is
emphasized when appropriate. However, such reactions are also
very interesting in their own right and are described below
irrespective of particular applications.

II. Water Splitting

Photochemical water splitting(1,7) can be viewed as the sim-
plest version of artificial photosynthesis:

2H 2 O!2H 2 þO 2 ; DH¼þ289kJ ð 7 Þ

PHOTOCHEMICAL ACTIVATION AND SPLITTING OF H 2 O 347