Chemistry Reference
In-Depth Information
classes based on either on a centred octagonal crown or a hexagonal chair shown at
the top of the figure. Although the centred hexagonal ring is not known as a distinct
species, it does give rise to the wide range of structures shown on the right hand side
of the figure, which add gold atoms either to the edges of the molecule or along the
symmetry axis. Adding single metal atoms or triangles of metal atoms along the
threefold axis leads to hemi-spherical or spherical clusters depending on whether
the addition occurs only on one side of the interstitial atom or both. Addition of a
single metal atom along the rotation axis above and below the interstitial atom leads
to a cube, and addition of triangles to these locations leads to an icosahedron. In
contrast additions of bridging metal atoms around the chair lead to flatter structures
which are described as toroidal or oblate. For the centred crown on the left hand side
of the figure, the toroidal chair structure is converted into a spherical structure by
squeezing together the four metal atoms which define planes above and below the
interstitial atom to form the square antiprism shown at the bottom left of the figure.
It is noteworthy that spherical and toroidal clusters with the same number of metal
atoms have polyhedral electron counts (pec) which differ by 2, e.g. spherical
[Au
9
(PPh
3
)
8
]
+
has a pec of 114, whereas toroidal [Au
9
(PPh
3
)
8
]
3+
has a pec of 112.
The polyhedral electron count (pec) is defined as the total number of valence
electrons donated by the gold atoms and the lone pairs donated to the surface gold
atoms by the phosphine ligands minus the total positive charge on the cluster. The
spherical clusters are therefore characterised by 12
n
+ 18 and the toroidal clusters
by 12
n
+ 16 electrons where
n
is the number of non-interstitial atoms [
6
,
11
,
13
]. If
the d shells of the gold atoms are excluded, the toroidal clusters have a sec count
(
skeletal electron count
) of 6 and the spherical clusters a sec count of 8. A molecular
orbital interpretation of this difference is discussed in some detail below in Sect.
3
.
The single crystal X-ray determinations suggested that the potential surfaces
connecting alternative toroidal or spherical structures are rather soft because for
closely related compounds alternative structures were observed. For example, for
[Au
9
{P(C
6
H
4
OMe)
3
}
8
]
3+
careful crystallisation led to crystalline modifications and
single crystal analyses showed one has the crown structure (shown on the top left
hand side of Fig.
3
) and the other a D
2h
structure previously observed for
[Au
9
{PPh
3
}
8
]
3+
(shown in the third row of Fig.
3
)[
45
,
46
]. These isomers may
be interconverted by applying high pressures [
46
].
31
P{
1
H} NMR experiments in
the solid state and in solution have confirmed that these clusters are generally
stereochemically non-rigid on the NMR time scale [
47
,
48
]. The majority of
transition metal carbonyl clusters of the later transition metals are stereochemically
rigid, and therefore this represents an important difference between the two classes
of compound which has to be accounted for by a reliable theoretical model
[
46
]. Although the gold clusters shown in Fig.
3
are homonuclear, a wide range
of cluster compounds with other interstitial metal atoms have been structurally
characterised and shown to follow a structural paradigm identical to that shown in
Fig.
3
[
8
,
49
]. The toroidal and spherical classification introduced above is also
applicable to these cluster complexes [
49
-
57
].
Higher nuclearity cluster compounds of the group 11 metals have also been
studied, and their structures have shown that they do not necessarily follow the
