Chemistry Reference
In-Depth Information
phosphine by smaller (and harder) anionic ligands. In general these degradation and
aggregation reactions are faster than those observed for metal carbonyl clusters of
the platinum metals. These studies also underlined the importance of the steric
requirements of the ligands, and this led to the definition of a cluster cone angle
which is closely related to the Tolman cone angle [
27
-
29
]. In recent years these
procedures have been elegantly extended by Konishi and others, who have shown
that growth and etching processes may be used to convert, for example,
[Au
6
(dppp)
4
](NO
3
]
2
into [Au
8
(dppp)
4
Cl
2
]
2+
and [Au
8
(dppp)
4
](NO
3
)
2
[
30
,
31
].
The replacements of tertiary phosphine by organothiolates have led to an
interesting range of molecular cluster compounds with 25-144 gold atoms, some
of which have proved to be sufficiently crystalline for definitive structural analyses.
The wide range of organothiolato-clusters results in part from their negatively
charged sulphur ligand which leads to stronger gold-ligand bonding and their
smaller cone angles which encourage larger cluster sizes. The syntheses and
structures of organothiolato-clusters have been discussed in some detail in the
previous volume [
32
,
33
]. It should be noted that the syntheses of phosphine and
organothiolato-clusters via reductive routes lead to mixtures and the use of
chromatographic and size separation techniques has had a major impact on reduc-
ing the dispersities and the isolation of atomically precise compounds which are
crystalline.
2 Single Crystal X-Ray Structural Determinations
The first single crystal X-ray structures of tertiary phosphine gold cluster com-
pounds were reported in the 1950s by Mason and McPartlin [
34
]. At that time they
represented the largest known cluster molecules of the transition metals. Since that
date numerous X-ray structural determinations have been reported and the relevant
literature up to 2000 was summarised by Mingos and Dyson [
35
]. The bond lengths
in these clusters are longer than that reported for the Au
2
dimer (2.472
) and
comparable to that which has been determined for the parent bulk metal which
adopts a face-centred cubic lattice (Au-Au ¼ 2.884
Å
). These bond lengths may be
related to a thermodynamic data, and it is noteworthy that the bond dissociation
enthalpy in the gold dimer is 228 kJ mol
1
, which is intermediate between Cl
2
(243 kJ mol
1
) and Br
2
(193 kJ mol
1
). The enthalpy of vaporisation for the bulk
metal is 343 kJ mol
1
, which is similar to that for copper 307 kJ mol
1
and greater
than that for silver 258 kJ mol
1
. Therefore, the metal-metal bond strengths in gold
clusters are anticipated to be reasonably strong and comparable with those in other
related cluster compounds of the transition metals. This suggests that gold is
capable of forming a wide range of cluster compounds particularly in low oxidation
states. The ligands PR
3
and SR
enhance the strengths of the metal-metal bond,
and the linear moiety Au-Au-L enhances s/d
z
2
hybridisation and encourages strong
radial bonding in clusters. This hybridisation and the relatively small contribution
Å
