to be roughly 1:2. Accordingly, a further Os complex should have
been formed. It was assumed to be [OsIII(NH 3 ) 5 (H 2 O)]^3 þ (see
below) which cannot be identified in the photolyzed solution by
its absorption spectrum. This spectrum consists only of a short-
wavelength shoulder at lmax¼220 nm of moderate intensity
(e¼1100). However, upon addition of iodide the aquo complex
undergoes a facile substitution yielding [OsIII(NH 3 ) 5 I]^2 þ( 66 ). This
complex shows a long-wavelength LMCT band (lmax¼407 nm,
e¼1970). Indeed, this absorption appears in the spectrum of the
photolyzed solution when iodide is added. The molar ratio ofIto
[OsIII(NH 3 ) 5 I]^2 þwas found to be nearly 3. NeitherInorIIwas
observed to react with iodide. Ammonia was detected as a further
photoproduct ofI. Any interference byIor the constituents of the
photolyzed solution could be excluded. The molar ratio ofIto
NH 3 was found to be approximately 2. Finally, the photolysis ofI
is not accompanied by the evolution of a gas. In contrast, the irradi-
ation of [OsII(NH 3 ) 5 (N 2 )]^2 þleads to the release of N 2 ( 67 ). In this
case at higher complex concentrations N 2 appears as gas bubbles
even at the beginning of the photolysis.
The long-wavelength absorption of I at 700 nm has been
assigned to an intervalence transition within the delocalized
OsII/OsIIIsystem while the UV bands have been attributed to
MLCT transitions terminating at the bridging N 2 ligand( 57 ).
These assignments are also reflected by the photoreactivity of
I. Light absorption by the 700 nm band is not associated with
any photoactivity. In contrast, MLCT excitation leads to the
oxidation of the metal and reduction of N 2 as anticipated in
Eq. (1). Of course, MLCT excitation and product formation do
not imply the transfer of six electrons to N 2 , but only the shift
of electron density owing to the covalent nature of the
complexes. However, the occurrence of this photoredox reaction
is certainly facilitated by the simple stoichiometry of the intra-
molecular photoredox process for the conversion ofItoII. In
distinction to I, MLCT excitation of [OsII(NH 3 ) 5 (N 2 )]^2 þ does
not result in the reduction of N 2 but leads only to the release
of N 2 ( 67 ) probably because a simple photochemical mechanism
yielding stable reduction products of N 2 is apparently not avail-
able in this case.
The distribution of the photoproducts ofIas obtained by ana-
lytical measurements suggests that the presence of an excess
electron in Eq. (1) causes complications.Itisreasonableto
assume that the primary photochemical step takes place
according to Eq. (33)since the reversal of this reaction type as
364 ARND VOGLER AND HORST KUNKELY