Chemistry Reference
In-Depth Information
Fig. 20 Vertex, face and fusion of cuboctahedral centred clusters
It was noted above that the centred icosahedral and the cuboctahedral Au
13
clusters have a very similar pattern of molecular orbitals, because of the predom-
inance of the radial metal-metal bonding. The tangential bonding-bonding inter-
actions are slightly larger for the icosahedron, because of the greater number of
nearest neighbour interactions. They are both characterised by 4 very stable skeletal
bonding molecular orbitals, which in the pseudo-spherical symmetry correspond to
[S
σ
]
2
[P
σ
]
6
. The cuboctahedron may also form more complex cluster architectures
by vertex, edge or face sharing as illustrated in Fig.
20
. These condensation
processes lead to a very similar pattern of molecular orbitals to that noted above
for icosahedra, and the relevant occupied skeletal molecular orbitals are
summarised at the bottom of Fig.
20
. Most importantly both classes of polyhedral
show a similar pattern of behaviour - the vertex sharing pair of clusters are
analogous to a pair of isolated clusters, and as the degree of condensation increases,
the number of skeletal molecular orbitals decreases until it reaches the limit for the
spherical cluster. The relevance of these conclusions for organothiolato-gold clus-
ters is discussed in more detail below.
The structure of the recently reported cluster [Au
14
(PPh
3
)
8
(NO
3
)
4
] (II) [
59
] has
similarities to the cuboctahedral fused structure discussed above since it is based on
two trigonal bipyramids joined by an Au-Au bond which is ringed by 4 gold atoms.
The molecule has approximately D
2h
symmetry which is the same point group as
the fused cuboctahedron. The skeletal molecular orbitals would follow the patterns
established in Figs.
18
and
19
. The sec of 10 electrons is consistent with the
