Chemistry Reference
In-Depth Information
Fig. 7 Frontier orbitals of AuL
n
(
n
¼ 1-3) fragments - the introduction of more
p
character is
introduced as the number of ligands increases, but the essential symmetry character of the frontier
orbital is retained [
78
]
tetrahedral edges is strongly bonding and the P
σ
, which is noded along half the
edges, is somewhat antibonding. At its simplest level a H¨ckel [
77
] interpretation
suggests that the bonding molecular S
σ
molecular orbital is stabilised by 3
(since it
has three nearest neighbour gold atoms) and each component of the P
σ
molecular
orbitals is destabilised by -
β
. The triple degeneracy of P
σ
leads to an equal
β
spreading of the
3
antibonding character. Therefore a stable closed shell
electronic configuration is associated with triangular [Au
3
L
3
]
+
and tetrahedral
[Au
4
L
4
]
2+
, which both have [S
σ
]
2
configurations. Although triangular clusters of
gold are not well established, there are several examples of tetrahedral gold
clusters, viz. [Au
4
(PBu
t
3
)
4
]
2+
, [Au
4
{P(C
6
H
3
Me
3
-1,3,5}
4
]
2+
, where each gold atom
is associated with a single phosphine ligand [
35
,
42
]. The stable [Au
4
]
2+
entity may
also be stabilised by a combination of phosphine ligands and anionic bridging
ligands along two edges of the tetrahedron, namely [Au
4
(
β
ʼ
-I)
2
(PPh
3
)
4
] and
[Au
4
(
-SnCl
3
)
2
(PPh
3
)
4
][
35
].
Lower nuclearity clusters of gold are sufficiently flexible and have sufficient
space to accommodate ML (or MX) and ML
2
(or MLX) ligand combinations,
because each of them has similar frontier orbitals which are able to participate in
metal-metal bonding (see Fig.
7
). Gold may also form trigonal ML
3
fragments, but
they are rarely observed within the clusters. The expansion of ligand sphere
involves some interactions with the gold 6p orbitals although they are rather high
lying, and this raises the energy of the frontier orbital but does not change the nodal
characteristics of the frontier orbital, which remains essentially a 6s/5d
z
2
hybrid
with small variations in 6p character. Therefore, gold clusters may be associated
ʼ
