8
54
H. Kunkely - A Vogler • Photooxidation of Dicyanoaurate(I)
of the irradiation the spectral changes are obscured the cyanide 7r* orbital is pushed below the empty
by a secondary photolysis. The primary photoprod- 6s orbital of gold and becomes the LUMO [13]
uct [Au(CN)2Cl2l~ undergoes a photosubstitution. of the complex. Accordingly, MLCT excitation of
After prolonged irradiation (Fig. 1) the absorption
[Au(CN)2]~ induces the photooxidation of Au(I)
spectrum approaches that of [AuCl4]_ (Amax = 323 to Au(III). Although the overall photoreaction is
nm, e = 5800; 227 nm, 44800) [15].
a two-electron oxidation of Au(I) it may initially
proceed by a one-electron oxidation generating an
Au(II) intermediate which either disproportionates
to Au(I) and Au(III) or is oxidized to Au(III) in a
second electron transfer step. In this context it is of
interest that the existence of Au2+ ions in different
systems even under ambient conditions has been
confirmed quite recently [18, 19].
The photoproduct [Au(CN)2Cl2]- undergoes a
further photolysis. The irradiation leads to a photo-
substitution which finally yields [AuCU]- . The na-
ture of the reactive excited state of [Au(CN)2Cl2]~
is not clear since low-energy LF as well as LMCT
states exist in close proximity [15]. In analogy to
other d8 metals such as Pt(II) [20], LF states are
certainly reasonable candidates for inducing substi-
tutions.
3. Discussion
The electronic spectrum of [Au(CN)2]- is char-
acterized by a set of MLCT absorptions [13]. At
room temperature solutions of [Au(CN)2]- do not
show any emission, and are not photoreactive in
the absence of a suitable electron acceptor. How-
ever, in the presence of oxygen a photooxidation
takes place which in the presence of chloride yields
[
Au(CN)2Cl2]~ as a well defined product [16].
In analogy to [AuC^]- and [AuBr2]~[3d] it is
suggested that the lowest-energy excited state of
[
Au(CN)2]~ undergoes electron transfer to oxy-
gen in the primary photochemical step. The low
quantum yield of photooxidation is certainly re-
lated to the small concentration of oxygen in so-
lution. The lowest-energy and reactive excited state
of [AuCl2]~ and [AuB^]- is of the metal-centered
ds type [3d, 17]. In contrast to these halide com-
plexes the lowest excited state of [Au(CN)2]~ is of
the MLCT type because by overlap with the gold 6p
Acknowledgment
Support of this research by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie is
gratefully acknowledged.
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