Chemistry Reference
In-Depth Information
A prolate distortion is favoured for clusters with [S
σ
]
2
[P
σ
]
2
electronic configu-
rations because the P
x
,
y
σ
components are destabilised relative to the P
z
σ
component
as shown on the left hand side of Fig.
8
. A prolate geometry is exemplified by the
[Au
6
(PPh
3
)
6
]
2+
(see II) cluster which has a pair of tetrahedra sharing a common
edge and [Au
6
(dppp)
4
]
2+
(see III) which has a structure based on a di-edge bridged
tetrahedron. The P
x
,
y
σ
components are localised on the atoms lying in the equatorial
plane, and the nodal plane leads to antibonding next neighbour gold-gold interac-
tions. A prolate distortion, which pushes the equatorial atoms closer amplifies the
splitting energy. In contrast an oblate distortion which pushes the polar atoms closer
together results in a greater destabilisation of the P
z
σ
component (see Fig.
8
).
The pentagonal bipyramidal cluster [Au
7
(PPh
3
)
7
]
+
has an oblate structure
(see IV) and indeed the shortest bond in the cluster is between the two apical
gold atoms. This structure is consistent with the closed sub-shell structure [S
σ
]
2
[P
x
,
y
σ
]
4
shown in Fig.
8
(right hand side). It is noteworthy that the distortions
described above create a sufficiently large HOMO-LUMO gap to ensure that the
clusters adopt a low spin state which is diamagnetic.
This analysis has proved sufficiently robust to account for the geometries of gold
clusters stabilised by soft ligands such as phosphines, iodide and SnCl
3
.Itis
important to recognise that the ligands play an important role in influencing the
stabilities and geometries of the clusters. In the absence of such ligands the bare
clusters either adopt rather different geometries or decompose to the bulk metal.
These structures of bare gold clusters have been established by careful spectro-
scopic studies in the gas phase and are supported by DFT calculations. The
structures of these clusters are discussed in some detail by Woodham and Fielicke
in the chapter “Gold Clusters in the Gas Phase” [
85
]. Their studies on clusters up to
Au
20
have shown that the smaller clusters generally favour close-packed planar
structures and the 2D to 3D transition occurs at Au
12
for anions. This has been a
puzzle for theory, but has now been correctly described, thanks to more recent
developments in density functional theory, which are able to handle dispersion
interactions. For metal carbonyl clusters, where radial and tangential interactions
are both significant, a larger number of bonding skeletal molecular orbitals result.
