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and core locations. The interconversion of staple ligands requires some of the gold
atoms to be sequestered from or added to the core. If the stoichiometry of the
[Au
m
(SR)
n
]
q
cluster is to be maintained, then this requires the loss of 2 [Au
2
(SR)
3
]
staple ligands to create 3 [Au(SR)
2
] staple ligands, an increase in the number of
gold atoms in the central core by one and the movement of two gold atoms from the
core to the surface to form the new S-Au dative bonds (see Fig.
23
). This creates a
problem for bonding models which relate the geometry of the core with a specific
number of metal atoms to the total number of electrons donated by the metal atom
and the ligands. DFT calculations also face some difficulties since they have to
model the migration of gold atoms to very different chemical environments,
i.e. from core to metallo-ligand.
These conceptual and practical difficulties may be circumvented by an alterna-
tive procedure which has as its starting point a specific example of a phosphine gold
cluster which has a well-defined geometry, i.e. an example drawn from Fig.
21
. The
chosen cluster, which has a well-defined stoichiometry and skeletal electron count,
is used to establish an isoelectronic series which is derived by replacing the
phosphines initially by thiolates (SR) and subsequently by [Au(SR)
2
] and
[Au
2
(SR)
3
] metallothiolato-ligands. Specifically the core cluster maintains its skel-
etal geometry by involving the same number of skeletal electrons (sec) across the
series, and it maintains the contribution from the ligating ligands constant by
replacing each phosphine ligand by an electron pair from the organothiolato-ligand,
e.g. two phosphine ligands are replaced by a bridging SR
ligand, which donates an
electron pair to each of the gold atoms it bridges (see Fig.
24
). The development of
an isoelectronic series is assisted because SR, [Au(SR)
2
] and [Au
2
(SR)
3
] are all
bidentate ligands and consequently all bond to two metal atoms and do so by
donating the same number of electrons. It is based on the following transformation
of SR
into the staple ligands shown in Fig.
25
:
þ
AuSR
!
Au
2
S
ð
3
SR
þ
AuSR
!
Au S
ð
2
:
ð
1
Þ
Therefore, the following general substitutional procedure may be proposed,
whereby the central core structure is retained throughout the series. The successive
addition of Au(SR) fragments shown above permits the development of a series of
clusters which have an identical Au
m
core with the same number of skeletal
electrons and the same number of gold atoms coordinated either to SR or the staple
ligands. The process is illustrated in Fig.
25
.
It may also be generalised as follows for homoleptic examples:
