inorganic chemistry

2 Cp 2 OsIII

hiþ

! Cp 2 OsIV

hi 2 þ

þCp 2 OsII ð 15 Þ

Accordingly, we suggest that the photolysis is preceded by a
reversible dimerization of the hydride complex that subsequently
undergoes a reductive elimination without the formation of
radicals:

2 Cp 2 OsIVðÞH

hiþ

!

Cp 2 OsIVHHOsIVCp 2

hi 2 þ

!

hn

Cp 2 OsIII
OsIIICp 2

hi 2 þ

þH 2

ð 16 Þ

This special type of a hydrogen bonding has also been considered
to occur in other cases( 15 ). Since this bonding is certainly rather
weak, the monomer is expected to prevail in the equilibrium. The
apparent quantum yield for the formation of [Cp 2 OsIIIOsIIICp 2 ]^2 þ
amounts tof¼ 10
^3 (lirr¼254 nm). However, it must be taken
into account that the photoactive dimeric hydride complex is
probably present only in small concentrations while the mono-
mer absorbs most of the light. A comparable photolysis has been
described earlier but without discussion of the mechanism:
2HIrI(PF 3 ) 4 !H 2 þ(PF 3 ) 4 Ir^0
Ir^0 (PF 3 ) 4 ( 16 ).
In coordinating solvents (L) such as CH 3 CN the Os(III) dimer
is well known to undergo a slow disproportionationEq. (17) ( 14 ):

Cp 2 OsIII
OsIIICp 2

hi 2 þ

þL!Cp 2 OsIIþ Cp 2 OsIVL

hi 2 þ

ð 17 Þ

A

b

a

1.6

0.8

0.0

220 320 420

l (nm)

520 620

FIG. 1. Spectral changes during the photolysis of 2.37 10
^4 M
osmocene in 5 M H 2 SO 4 under argon at room temperature after 0 min
(a), 20, 40, and 80 min (b) irradiation times withlirr¼254 nm 1-cm cell.

350 ARND VOGLER AND HORST KUNKELY