Chemistry Reference
In-Depth Information
D
y
z
s
D
xz
s
D
z2
s
P
y
s
P
x
s
P
z
s
S
s
Fig. 19 Skeletal molecular orbitals for an icosahedron and a pentagonal bipyramid which has
19 metal atoms, 2 of which occupy interstitial sites
orbitals are illustrated in Figs.
18
and
19
. The most spherical and most closely
packed M
19
cluster therefore behaves analogous to the centred icosahedron since
both clusters are associated with four [1S
σ
]
2
[1P
zσ
]
2
[1P
x,y
σ
]
4
skeletal molecular
orbitals although the former has two interstitial atoms. This behaviour is reminis-
cent of the united atom approach which interrelated the molecular orbitals of
diatomic molecules to the atomic orbitals of a single atom. The orbitals are
correlated using symmetry arguments and are required to obey the non-crossing
rule. The number of bonding skeletal molecular orbitals in carbonyl clusters of the
earlier transition metals has been analysed using Tensor Surface Harmonic Theory
[
77
], and they show a similar pattern of behaviour. For example, vertex, edge and
face sharing octahedra have the following number of skeletal molecular orbitals:
11, 9 and 7 and the most condensed example has the same number of skeletal
molecular orbitals as the parent octahedron [
71
,
72
].
Although a gold phosphine cluster with face sharing or fused icosahedral
structures have not been characterised to date, the corresponding and isostructural
platinum carbonyl cluster [Pt
19
(CO)
17
]
8
has been structurally determined [
90
]. It
is noteworthy that gold clusters based either on a pair of face sharing icosahedra or
the fused geometry have been stabilised by organothiolato-ligands recently. They
have been structurally characterised and are discussed in more detail below.
