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the strongly acidic condition, while the use of diphosphine ligands with a
longer alkyl bridge (dppp, dppb, dpppe) results in the preferential formation of
[Au
13
Cl
4
(P^P)
4
]
3+
(41-43) (Scheme
7
).
3.2.6 Redox Reactions
As mentioned in the above paragraphs, the addition of nucleophiles such as Cl- to
monophosphine-coordinated clusters promotes the aggregation-induced growth of
the cluster core. However, we recently found that the nucleophilic reaction towards
[Au
8
(dppp)
4
]
2+
(9) proceeds with the retention of the nuclearity but accompanies
two-electron autoxidation and rearrangement of the cluster skeleton[
91
]. For
instance, the reactions with nucleophiles such as halide and acetylide anions afford
[Au
8
(dppp)
4
X
2
]
2+
CPh (12)), where the charge of the Au
8
skeleton is altered from 2+ to 4+ (Scheme
8
). In this relation, electrochemical
two-electron redox processes between [Au
9
(PPh
3
)
8
]
3+
(14) and [Au
9
(PPh
3
)
8
]
+
,
which accompany the skeletal rearrangement from toroidal to spherical, have also
been reported (Scheme
9
)[
32
,
92
]. The large structural change may reflect the soft
potential energy surface of this class of compounds.
(X
¼
Cl (11), X
¼
C
3.2.7 Summary
A variety of PGCs can be synthesized in good yields when the appropriate reagents
and pathways are used. However the cluster products are generally unpredictable
and vary significantly with the subtle difference of synthetic parameters (methods,
conditions, reagents, and so on).
4 Geometrical Structure
As noted in Sect.
2
, single-crystal X-ray crystallography is the sole definitive
method to elucidate the 3-dimensional structural features. PGCs with a nuclearity
of 5, 6, 7, 8, 9, 10, 11, 13, 14, 20, 22, 24, 25, and 39 have been structurally
identified. For selected examples, geometrical descriptions and Au-Au bond dis-
tances are summarized in the nuclearity order (Tables
1
and
2
). Centered polyhedral
geometries are generally favored because the center-to-peripheral interactions
effectively enhance the stability of cluster skeleton. The majority of the clusters
with
N
8 fall into this category. However, the use of sterically hindered ligands
or chelating diphosphines sometimes allows the generation of exceptional struc-
tures that cannot be described only with polyhedra.
