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appears to assist the holding of the cluster skeleton. Wang et al. reported that
Au
22
(dppo)
6
(37), although its charge is ambiguous, has a structure composed of
two quasi-icosahedral Au
11
units fused via four short Au-Au bridges [
73
]. There
are also examples of the utilization of thiolate as bridging ligands. Shichibu
et al. demonstrated that the structure of [Au
25
(PPh
3
)
10
(SC
2
H
5
)Cl
2
]
2+
(39) cluster
can be defined as two Au
13
cluster units joined through five thiolate bridges by
vertex sharing [
85
]. An analogous thiolate-bridged PGC composed of two incom-
plete icosahedral Au
12
units, in which one vertex atom of Au
13
icosahedron is
missing, has been recently reported by Jin et al. for [Au
24
(PPh
3
)
10
(SC
2
H
4
Ph)
5
X
2
]
+
(X
¼
Br or Cl, 38)[
79
]. The Au-Au bond distances in these icosahedron-based
clusters (36-39) are similar to those of smaller centered clusters, being in the range
2.63-3.23 Å (Table
2
). Short distances less than 2.7 Å are observed for the center-
peripheral radial bonds.
On the other hand, the structures of [Au
39
(PPh
3
)
14
Cl
6
]Cl
2
40·Cl) is exceptional,
being defined as a 1:9:9:1:9:9:1 layered hcp/hcp
0
, which is virtually different from
the icosahedron-based structure.
4.2 Non-centered Polyhedral Clusters
Lower nuclearity clusters favor condensed polyhedra rather than centered geome-
tries (Fig.
4
). For hexanuclear clusters (
N
6), the simplest geometry is octahe-
dron. Bellon et al. reported an octahedral structure for [Au
6
{P(p-Tol)
3
}
6
]
3+
[
48
],
but later it is identified to be a carbon-centered octahedron [
100
]. Very recently
Echavarren et al. reported organometallic Au
6
clusters protected by ortho-aurated
PPh
3
ligands [Au
6
(PPh
2
C
6
H
4
)
4
](SbF
6
)
2
(2·SbF
6
), though their gold atoms are
formally monovalent, adopt an octahedron [
93
]. In this structure, the octahedron
is severely distorted, where some edges are short in the range 2.72-2.75 Å but the
other bonds are longer than 3.10 Å.Au
6
cluster can also adopt tetrahedron-based
geometries. Mingos et al. reported an edge-sharing bitetrahedral structure of
[Au
6
(PPh
3
)
6
]
2+
(3)[
40
], where the shared (2.652 Å) and terminal (2.662 and
2.669 Å) edges are significantly shorter than the other eight edges (2.762-
2.839 Å) (Table
3
). We have recently shown that an edge-sharing Au
4
tetrahedron
trimer ([Au
8
(dppp)
4
]
2+
, 9), which is an extended version of 3, has a similar
structural feature [
81
]. The bond distances of two shared and terminal edges are
2.623 and 2.607 Å, respectively, which are again shorter than the distances of the
remaining connecting bonds (2.824-2.896 Å) (Table
3
). A face-sharing
tritetrahedral structure has been reported for [Au
6
(xy-xantphos)
3
]Cl (5), where
the edge shared by three Au
4
tetrahedra shows a short distance (2.653 Å)
[
77
]. Thus, in the structures based on Au
4
tetrahedron, the short distances are
found for the shared and terminal edges.
¼
