irradiated. Does the photolysis take place according to the simple
Eq. (37)?
ðÞNH 35 OsIIIðÞm-N 2 OsIIIðÞNH (^35)
hi 6 þ
!2 OsVIðÞNH 34 N
hi 3 þ
þ2NH 3
ð 37 Þ
Unfortunately, [(NH 3 ) 5 OsIII(m-N 2 )OsIII(NH 3 ) 5 ]^6 þ is rather labile
and undergoes a facile decomposition in aqueous solution even
at 5C(57,70). Accordingly, a detailed study is very difficult to
perform, but a qualitative experiment is quite revealing. In dis-
tinction to the photolysis of [(NH 3 ) 5 OsII(m-N 2 )OsIII(NH 3 ) 5 ]^5 þ, the
irradiation of [(NH 3 ) 5 OsIII(m-N 2 )OsIII(NH 3 ) 5 ]^6 þ is accompanied
by the vigorous evolution of nitrogen which takes place also ther-
mally but with a much smaller rate. The photochemical behavior
of the 6þion can be explained by its excited state properties. It is
well known that MLCT transitions of Os(III) complexes occur at
much higher energies than those of Os(II) ( 71 ). A reactive
(Os!N 2 ) MLCT state of the 6þion is apparently not anymore
accessible by conventional light sources. As an alternative or
additional effect, LF states of the 6þ ion are now populated.
These LF states initiate the release of N 2 in agreement with
the general behavior of LF states.
While the photoactivation of dinitrogen in [(NH 3 ) 5 OsII(m-N 2 )
OsIII(NH 3 ) 5 ]^5 þin an aqueous solution leading to the reductive
cleavage of N 2 has been achieved it is of considerable importance
if the nitride ligand of the photoproduct can be utilized for the
formation of useful nitrogen compounds( 56 ). Although the study
of further reactions of [OsVI(NH 3 ) 4 N]^3 þ was not within the
scope of the present investigation, it is well known that OsVIN
complexes are able to undergo nitrogen atom transfer reaction
or reduction to OsIIor OsIII(69,72,73). In the latter case, proton-
ation of the nitride ligand yields NH 3. In this sense, our
observations can be directly related to the Haber–Bosch process,
which takes place at a solid interphase. In this context, it is of
interest that Schrauzer ( 74 ) and Kisch ( 75 ) observed the photore-
duction of N 2 at TiO 2.
In summary, the photolysis of aqueous [(NH 3 ) 5 OsII(N 2 )
OsIII(NH 3 ) 5 ]^5 þinduced by MLCT excitation leads to the reduc-
tive splitting of the bridging N 2 ligand yielding [OsVI(NH 3 ) 4 N]^3 þ
as main photoredox product. It should be emphasized that the
electronic CT excitation does not only provide a suitable intramo-
lecular redox reaction but may supply also the necessary energy
for activation and cleavage of the very stable N 2 molecule.
366 ARND VOGLER AND HORST KUNKELY