Chemistry Reference
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Fig. 4 Linking of icosahedral polyhedral observed in higher nuclearity gold-silver clusters. The
interstitial atoms at the centre of each icosahedron have been omitted
close-packed arrangement found in the bulk metal, but have structures which result
from the sharing of vertices, edges and faces of simpler polyhedral moieties,
e.g. octahedron and cuboctahedron. This general observation is consistent with
the view that cluster growth is a kinetically controlled process which may result in
the trapping of less symmetrical structures based on the condensation of simpler
polyhedral species and particularly the cuboctahedron and the icosahedron. Teo and
his co-workers have structurally characterised a number of vertex sharing icosahe-
dral clusters containing silver and gold atoms [
21
,
58
], which are illustrated in
Fig.
4
. The parent icosahedral gold cluster ion [Au
13
Cl
2
(PMe
2
Ph)
10
]
3+
[
35
] was
predicted on the basis of theoretical calculations in 1976 and synthesised and
structurally characterised in 1981. The related gold-silver clusters
[Au
13
Ag
12
Br
8
{P(p-tol)
3
}
10
]
+
and [Au
22
Ag
24
Br
10
{PPh
3
}
12
] and gold-silver-
platinum clusters [Pt
2
Au
10
Ag
13
Cl
7
{P(p-tol)
3
}
10
]
+
have been structurally
characterised and are illustrated in Fig.
4
. Interestingly, and for reasons, which
are discussed in more detail below, the clusters have skeletal electron requirements
which represent a sum of those for the parent icosahedron. Icosahedral and
cuboctahedral polyhedra are also important building blocks in organothiolato-
gold clusters.
Recently Simon and his co-workers [
59
] have reported that Malaesta's original
synthesis of [Au
9
(PPh
3
)
8
](NO
3
)
3
also provides, albeit in low yield a novel, but
closely related, cluster with the formula [Au
14
(PPh
3
)
8
(NO
3
)
4
]. It has two trigonal-
bipyramidal Au
5
fragments directly connected by a short Au-Au bond (2.651
),
and this bond also has a collar of 4 AuNO
3
fragments leading to an ellipsoidal shape
and a polyhedron having triangular faces (see Ia). The radial Au-Au bond lengths
are shorter than the surface gold bond lengths. Simon et al. have proposed that the
structure of the complex may be interpreted in terms of an adaptation of a
Å
