Chemistry Reference
In-Depth Information
-
+
-
+
+
P
s
xy
-
P
s
z
P
s
xyz
-
-
+
+
+
P
s
xy
P
s
z
-
S
s
S
s
S
s
Spherical
Prolate
Distortion
Oblate
Distortion
Fig. 8 Effect of prolate and oblate distortions on the energy levels of the particle in a sphere
problem
with 0, 1 or 2 donors, and this affects the total cluster electron count although the
number of skeletal bonding molecular orbitals remains the same. In such cases it is
less confusing if the number of electron involved in skeletal bonding (
sec
)is
identified, i.e. in these tetrahedral clusters, [S
σ
]
2
defines the nodal characteristics
of the molecular orbital and the number of electrons involved in metal-metal
bonding [
78
]. As indicated above additional ligands may be involved in bridging
situations, e.g. [Au
4
(
-SnCl
3
)
2
(PPh
3
)
4
], or may incorporate
another gold atom, e.g. [Au
5
(dppm)
3
(dppm-H)]
2+
, which has a gold containing
molecule acting as a tridentate ligand (see Ia). This provides perhaps the simplest
example of a “staple” ligand, which has proved to be important such a distinctive
feature of the structures of organothiolato-gold cluster [
32
,
33
].
[Au
n
L
n
]
m
+
clusters (
n
¼ 5-7) have a set of four skeletal molecular orbitals
which are potentially available for bonding, i.e. S
σ
and P
σ
and (
n
4)D
σ
antibonding
skeletal molecular orbitals. For the complete occupation of the S
σ
and P
σ
shells,
8 electrons are required, and this would lead to anionic gold phosphine clusters
where the negative charges would be destabilising, e.g. [Au
5
(PPh
3
)
5
]
3
,
[Au
6
(PPh
3
)
6
]
2
,[Au
7
(PPh
3
)
7
]
. Partial filling of the P
σ
shell reduces the negative
charge and can lead to a closed sub-shell (and a diamagnetic ground state) if a
distortion occurs which removes the degeneracy of the P
σ
shell. This can be
achieved by undergoing prolate and oblate distortions as shown in Fig.
8
. The
relationship between the distortion and the orbital splitting may be analysed using
perturbation theory methods based on either molecular orbital or crystal field
models. The results are summarised in Fig.
8
[
82
,
84
].
ʼ
-I)
2
(PPh
3
)
4
] and [Au
4
(
ʼ
