Chemistry Reference
In-Depth Information
have a dissociation energy of 2.5 eV (241 kJ mol
1
) which is slightly larger than
2.3 eV (228 kJ mol
1
) dissociation energy of Au
2
. They have also explored the
conformations of the complexes and shown that these conformations may influence
the S-Au-S bond angle adopted when the ligands are stapled to the gold core. The
wider (124
) angle is observed in larger clusters and a smaller angle of 100
is
observed in smaller clusters. The series of staple ligands [Au
x
(SR)
x
+1
] are probably
in equilibrium in the solution surrounding the growing gold cluster molecule and
provide a flexible source of those ligands which have just the right bite angle and
spanning dimension for a specific gold cluster. Their presence therefore reduces the
dispersity of the mixture of clusters present at the end of the reaction. The dative
bonds between the sulphur atoms of the gold thiolato-oligomers and the central gold
core may be supplemented by aurophilic interactions which are represented sche-
matically in Fig.
22
by dotted lines.
If formal oxidation formalisms are used (i.e. Au
+1
and SR
) to assign electrons to
the staple ligands, then the cluster molecules may be assigned the general formula
{[Au]
a
+
a
0
}
x
+
{[Au(SR)
2
]
b
[Au
2
(SR)
3
]
c
(SR)
d
}
x
:
a
+
a
0
represents the total number of
core gold atoms,
a
represents those gold atoms which are unavailable for bonding to
the S ligands either because they are interstitial atoms or a surface atoms which lie in
concave surface regions making them inaccessible for gold-sulphur bonding and
a
0
represents the number of gold atoms of the central core which are capable of forming
dative bonds with the staple ligands [
106
-
112
]. Table
1
gives specific examples of
this notation. The formalism represents a reasonable formal partitioning of charges
between the positively charged central core and the negatively charged surface
staple motifs. Since the Au
+1
ion has an empty 6s shell, it does contribute to the
skeletal molecular orbitals of the central gold kernel. Recently a reversal of this
formal charge separation has been proposed for cationic staple cadmium bromide
ligands (see, e.g. Fig.
22
), which have been observed to stabilise anionic platinum
metal carbonyl anionic clusters, e.g.
[Pt
19
(CO)
17
]
8
stabilised by {[Cd
5
(-
-Br)
5
Br
2
(dmf)
3
]
3+
. In this case the staple ligands are cyclic [
90
].
In the last decade, DFT calculations have been used increasingly to rationalise
the observed structures of clusters and to use the information obtained to develop
models to predict the stoichiometries and structures of new clusters with a good
degree of accuracy. Years of chemical experience suggests that extra computing
power is most effectively used when combined with intelligent qualitative and
semi-quantitative models, which can provide an interpretation which may be used
imaginatively by synthetic chemists.
The key structural determinations of [Au
25
(SCH
2
CH
2
Ph)
18
]
and
[Au
102
(p-MBA)
44
] and subsequent studies have led to the following generalisations
[
91
,
96
]:
ΚΌ
1. The central gold core is approximately spherical and approximates to close
packing, and cuboctahedral and icosahedral fragments are common components,
e.g. an icosahedral Au
13
core in the former and a decahedral D
5h
Au
79
core in the
latter.
2. The staple gold thiolato-ligands are bonded to the central core using Au-S bonds
and in general each atom on the outer face of the core is bonded to a single S
