inorganic chemistry

generated in the singlet state, at least partially. When the pho-
tolysis is carried out in acetonitrile in the presence of
diphenylacetylene, benzil was formed and identified by its char-
acteristic luminescence atlmax¼507 nm. The addition of^1 O 2 to
diphenylacetylene has been previously used for the detection of
singlet oxygen ( 21 ).
We suggest that the photolysis of the hydroxo complex consists
of the following consecutive steps:

Cp 2 OsIVðÞOH

hiþ

>Cp 2 OsIV¼¼OþHþ ð 20 Þ

2Cp 2 OsIV¼¼O

!

Cp 2 OsO 2 OsCp 2 ð 21 Þ

Cp 2 OsO 2 OsCp 2!

hn

2Cp 2 OsIIþO 2 ð 22 Þ

The electronic structure of the fragment [OsO 2 Os]^4 þis not clear.
However, we assume that this binuclear cation exists as peroxo
complex, Cp 2 OsIII(O 22
)OsIIICp 2. Binuclear peroxo complexes of
this type frequently photolyze under elimination of oxygen in
general and^1 O 2 in particular( 22 ).
Recently, we have studied such a photoredox reaction( 6 ) that
proceeds in analogy to the photolysis of the binuclear osmium
complex Eq. (23):

ðÞporphyrinFeIII O 22




FeIIIð Þ!porphyrin

hn=LMCT

2 porphyrinðÞFeIIþO 2 1 O 2

 ð^23 Þ

In this context, it is of interest that the complex [Cp 2 OsIII(S 22
)
OsIIICp 2 ]^2 þis apparently not stable but thermally decomposes
to osmocene and sulfur( 23 ). In any case, the product formation
is certainly facilitated by the high stability of osmocene.
In summary, the photochemical water splitting can be
described in a simplified version by the cyclic process shown in
Scheme 1.
The protonation of osmocene (Eq. 12) can be viewed as an oxi-
dative addition leading to the formation of Os(IV). Since Os(IV) is
an oxidant, the complex Cp 2 OsIVHþshould be characterized by a
(H
!OsIV) LMCT transition at relatively low energies.
Although a corresponding absorption band was not identified
the photolysis of the hydridic complex (Eq. 12) seems to originate
from such a LMCT state. Moreover, [Cp 2 OsIV(OH)]þ
should have available a low-energy (OH
!OsIV) LMCT state.
The corresponding LMCT absorption of the hydroxo complex or
its deprotonated form Cp 2 OsIVO appears at lmax¼390 nm.

352 ARND VOGLER AND HORST KUNKELY