Chemistry Reference
In-Depth Information
compounds of the group 11 metals, although similar conclusions may be derived for
these metals by using a fragment molecular orbital mode of analysis, which utilises
the fundamental results of perturbation theory.
For example, there are a series of gold clusters based on edge-linked tetrahedra,
which are illustrated in (V)-(VII). The initial tetrahedral structure was established
by Steggerda, Mingos and Schmidbaur, the bi-tetrahedron by Mingos and the
tri-tetrahedron by Kominishi and his co-workers [
86
-
89
], who have also studied
the UV-visible spectral properties of these ions. The series of clusters share a 2+
charge, and this suggests a 2e increment in the sec as pairs of gold atoms are added.
Starting with the molecular orbitals of the tetrahedron which were introduced in
Fig.
6
, it is possible to evaluate the effect of adding successive tetrahedra by sharing
edges and thereby increasing the number of metal atoms by 2. The problem has
close similarities to the linking of hydrogen atoms in a linear chain since the parent
tetrahedron is characterised by a single in-phase molecular orbital [S
σ
] (see Fig.
6
).
The problem is also analogous to the classical quantum mechanical solution to the
particle in a box problem which results in the introduction of successive nodes as
the principle quantum number is increased. The number of bonding skeletal molec-
ular orbitals increases by one for each pair of gold atoms added, and the resultant
molecular orbitals are illustrated in Fig.
15
. In the notation which we have intro-
duced above the relevant molecular orbitals are given the pseudo-atomic symbols
[S
σ
]
2
for Au
4
,[S
σ
]
2
[P
σ
]
2
for Au
6
and [S
σ
]
2
[P
σ
]
2
[D
σ
]
2
for Au
8
. This notation
emphasises the number of nodal planes which result from the successive addition
of pairs of gold atoms to generate the chain structures. The analysis is consistent
with the observed electron counts and may also be used as a basis for interpreting
their optical spectral characteristics [
86
-
89
]. The chain of tetrahedra has molecular
orbitals which mimic those of alternant hydrocarbons which are characterised by a
non-bonding molecular orbital when there are an odd number of carbon atoms. The
occurrence of such non-bonding molecular orbitals results in related cationic,
radical and anionic species, and it is possible that the gold chains may show similar
redox properties for odd numbers of tetrahedra.
For the group 11 metals a number of structures have been determined which
have several icosahedra either linked in a line or joined in a triangle or tetrahedron.
Teo and his co-workers have been particularly active in developing this area and
