Chemistry Reference
In-Depth Information
from the gold 6p orbitals is associated with the large relativistic effects associated
with gold - an effect which has been reviewed extensively by Pyykk¨ [
36
-
38
]. The
great majority of cluster compounds of gold have a formal average oxidation state
which lies between 0 and +1 which corresponds to a partially filled 6s orbital in its
valence shell of
{5d
10
}{6s}
x
, where
x
¼ 0.5 in [Au
4
(PR
3
)
4
]
2+
to 0.20 in
[Au
39
(PPh
3
)
12
Cl
6
]
2+
. Linear metal-ligand bonding is a characteristic of gold
(I) compounds and is also a feature of gold cluster compounds. There are some
rare examples of compounds with gold-gold bonds in the +2 oxidation state, but
they are relatively rare. Gold-gold bonding is also a feature of alloys of gold with
the alkali metals where the formal oxidation state of gold lies between
1 and
0[
39
].
The majority of gold(I) complexes have a linear geometry and the hybridisation
at the metal is approximately to sd
z
2
. Many of these linear gold compounds show
additional gold contacts to other gold atoms, which are significantly longer than the
metal-metal distance in the metal (2.88
...
), but shorter than those expected from the
Å
sum of the van der Waals' radii (3.60
). It has been suggested that these aurophilic
interactions may contribute as much as 46 kJ mol
1
[
40
], but generally are
estimated to be approximately 20-30 kJ mol
1
. In smaller gold clusters which do
not have a central gold atom the average gold-gold bond lengths ranges from 2.76
to 2.87
Å
, but in larger clusters with an interstitial gold atom, the bond lengths
involving the central atom are generally shorter than those on the periphery, e.g. in
Au
8
clusters, 2.59-2.77
Å
for the latter. In
icosahedral Au
13
phosphine clusters the centre to periphery distances are 2.716
(2)-2.789(2)
for the former and 2.83-2.96
Å
Å
, whereas the peripheral gold-gold bond lengths lie between 2.852
(3) and 2.949(3)
Å
. The observation of relatively strong radial bonds in these
compounds led to some confusion in the 1970s as to whether these compounds
were best described as cluster compounds or coordination compounds, and the term
“porcupine” clusters was coined in order to emphasise the anisotropy in the metal-
metal bonding [
41
]. [Au
39
Cl
6
(PPh
3
)
14
]Cl
2
which has a structure based on hexago-
nal close packing the central interstitial 12 coordinate gold atom has gold-gold
bond lengths which average 3.040
Å
, and the gold-gold bond lengths in successive
hexagonal layers range between 2.792 and 2.882
Å
[
21
]. This pattern of bond
lengths does not follow that noted above for simpler phosphine clusters and remains
somewhat surprising.
Molecular phosphine gold cluster molecules and ions with 4-7 metal atoms have
been structurally characterised and their structures are summarised in Fig.
1
[
35
]. The tetrahedron is a common building block and is observed in the 4, 5, and
6 atom clusters. The seven-atom cluster has a pentagonal bipyramidal structure. It is
noteworthy that this structure has a very flat geometry leading to a short Au-Au
interaction (2.58(2)
Å
) between the axial gold atoms. The equatorial-equatorial
and axial-equatorial gold-gold bonds average 2.9
Å
respectively. The
absence of a symmetric octahedral Au
6
structure is noteworthy. The cavity at the
centre of the tetrahedron, trigonal bipyramid and octahedron are sufficiently small
and 2.8
Å
Å
