Chemistry Reference
In-Depth Information
polyhedral molecules [
69
,
70
]. The TSHM when combined with group theoretical
analyses led to a clearer understanding of apparent exceptions to the electron
counting rules which were formalised in the polyhedral skeletal electron pair theory
(PSEPT) developed primarily by Wade and Mingos [
10
,
14
,
65
,
71
-
81
]. This model
was subsequently extended to condensed and high nuclearity clusters [
71
-
73
].
According to the PSEPT the polyhedral geometry is influenced primarily by the
total number of valence electrons in the cluster molecule. This is determined by the
number of bonding and non-bonding molecular skeletal molecular orbitals, which
reflects the geometry of the cluster, and the number of molecular orbitals formed by
the overlap of ligand and metal molecular orbitals. The Tensor Surface Harmonic
Theory underlined the fact that the relationship between isostructural transition
metal and main group clusters originates because they both share an identical set
of antibonding molecular orbitals which are unavailable for bonding, because they
are strongly metal-metal antibonding and are unable to accept electron pairs from
the ligands [
74
-
77
]. For rings and three-connected polyhedral molecules, the results
are exactly the same as those predicted by the noble gas rule. This connection is lost
for four-connected and deltahedral polyhedral molecules because the relevant metal
carbonyl and main group fragments use only three orbitals for skeletal bonding and
the one-to-one relationship between edges and orbitals is lost. The majority of
transition metal clusters are stereochemically rigid as far as their metal skeletons
are concerned although the ligands on the surface of the cluster may migrate rapidly
over the surface of the cluster. From an early stage it was apparent that the later
transition metals and particularly palladium and platinum and silver and gold did not
conform to the generalisation embodied in PSEPT. This led to specific calculations
on platinum and gold clusters which accounted for their exceptional behaviour, and
this led to the prediction of the electronic requirements for gold clusters with
interstitial main group, e.g. octahedral [Au
6
C(PPh
3
)
6
]
2+
, and group 11 metal atoms
[
43
,
44
,
77
], e.g. icosahedral [Au
13
(PR
3
)
12
]
5+
[
77
]. As the number of structures of
gold clusters increased and NMR studies were completed, it became apparent that
many of the higher nuclearity gold clusters were stereochemically non-rigid in
solution and displayed alternative isomeric structures in the solid state. The accep-
tance that these clusters were interconnected by soft potential energy surfaces
suggested that it was not appropriate to classify these structures in a rigid manner
which had been developed for clusters of the earlier transition metals, which
conform to PSEPT, and it is more appropriate to classify them according to whether
they adopt spherical, prolate or oblate (toroidal) topologies [
10
,
14
,
78
-
84
].
3.2 Bonding in Simple Gold Clusters
The bonding in molecular cluster compounds of gold was initially based on rather
unsophisticated semi-empirical molecular orbital calculations. These calculations
led to some interesting insights into the bonding in phosphine gold clusters. These
initial and by today's standards primitive theoretical analyses worked remarkably
well for gold because the bonding is dominated primarily by the 6s valence orbital
