inorganic chemistry

Finally, LMCT excitation leads to the reduction to Os(II) and oxi-
dation of hydroxide, oxide, or peroxide to oxygen.
It is apparent fromScheme 1 that the reductive and oxidative
part of the water splitting by osmocene shows interesting
analogies to photosystems I and II of natural photosynthesis
which also take place as separate processes.

III. Carbon Dioxide Splitting

For artificial photosynthesis most efforts have been devoted to
photochemical water splitting (Eq. 7) while other possibilities
have also been considered. Natural photosynthesis

CO 2 þH 2 O!

1

n

ðÞCH 2 OnþO 2 ; DH¼þ470kJ ð 24 Þ

yields carbohydrates as energy-storing products (Eq. 24). Sur-
prisingly, very little is known about photosplitting of CO 2 Eqs. (2)
and (4) as another simple version of artificial photosynthesis.

CO 2 !COþ

1

2

O 2 ; DH¼þ238kJ ð 25 Þ

CO 2 þH 2 O!COþH 2 O 2 ; DH¼þ281kJ ð 26 Þ

Equation (26) can be easily transformed to Eq. (25) by dispropor-
tionation of hydrogen peroxide. It is quite interesting that the
products of reaction (25) have been labeled as carbon monoxide
detonating gas more than 70 years ago ( 24 ). This emphasizes also
the analogy of splitting water (Eq. 7) and carbon dioxide (Eq. 25).

[Cp 2 OsIV(H-)]+

[Cp 2 OsIV]2+

[Cp 2 OsIVOH]+

1/2H 2

hn

OH-

hν

H+ + 1/2O 2 H

+

Cp 2 OsII

SCHEME1. Photochemical water splitting.

PHOTOCHEMICAL ACTIVATION AND SPLITTING OF H 2 O 353