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546 Metal Particles and Cluster Compounds

teins, and many synthesized model complexes do not pos-
sess extensive metal-metal bonding. The lack of M M
bonding in these clusters may be due to the low Fe:S ra-
tios. The abundance of sulfide ligands makes these clusters
electron rich. Perhaps population of M M antibonding
orbitals is the result.
Our brief look at atomic donor ligands will conclude
with the halogens. The reluctance of a halogen to share
all seven of its valence electrons with a cluster is made
evident by the rarity of cluster compounds which contain
an interstitially bound halogen. As a surface ligand halo-
gens may be terminally bound to a metal atom, in which
case it is serving as a one-electron donor, or they may act
as three-electron donors by adopting a bridging mode of
coordination. Both µ 2 - and µ 3 -bridges are common.
This group of atomic donor ligands is unique in that they
stabilize a host of binary cluster compounds. In fact, the
first compounds recognized to have a cluster framework
were octahedral metal chlorides. Two common forms of
octahedral metal halide clusters exist (Fig. 39). One type
is seen in [(Ta 6 (µ 2 -Cl) 12 ))Cl 6 ] 4− where all twelve edges
of the tantalum octahedron are bridged by chlorine atoms.
The other six chlorine atoms are terminally bound, one to
each Ta vertex, and may be removed to produce the di-
FIGURE 37 Structure of (a) [Rh 10 (µ 8 -S)(CO) 22 ] 2− and (b)
cation [Ta 6 (Cl 12 )] . [(Mo 6 (µ 3 -Cl) 8 )Cl 6 ] 2− exhibits the
2+
[Rh 17 (µ 9 -S) 2 (CO) 32 ] 3− . The Rh 10 cluster is a bicapped square
antiprism. A sulfido ligand occupies the square antiprism cavity. second common structural form where all eight faces of
The Rh 17 cluster contains three square antiprisms. The cavity of the octahedron are capped by triply bridging chlorides.
the center one is occupied by an Rh atom. The two outer square The six-terminal chlorides in this cluster may also be
antiprisms each encapsulate a one sulfur atom. (Terminal CO are
not shown.) removed.
A precursor of the prevalent [(M 6 (µ 3 -X 8 )X 6 ] 2− core is
2−
seen in [(Mo 5 (µ 3 -Cl) 4 (µ 2 -Cl) 4 Cl 5 ] . This is related to
One such model recently synthesized is [Fe 8 S 6 I 8 ] 3− [(Mo 6 (µ 3 -Cl) 8 )Cl 6 ] 2− by the removal of a Mo–Cl vertex
(Fig. 38). This cluster is unique in that the Fe 8 core forms (this is a conceptual relationship and not one achieved
an almost perfect cube. The six faces of the cube are chemically). The nido Mo 5 cluster contains four (µ 2 -Cl)
capped by (µ 4 -S) ligands and each Fe vertex has one ligands which would be µ 3 -chlorides if the sixth vertex of
terminal iodide ligand. Although the Fe Fe distances of the octahedron were in place.
˚
2.71–2.73 A in this cluster supports the presence of metal– The bulk of binary metal–halide clusters involve the
metal bonding many of the naturally occurring Fe–S pro- early transition metals, however, later transition metal–
halide clusters are known. [Pt 6 (µ 2 -Cl) 12 ] is one example.

FIGURE 38 Structure of Fe 8 S 6 I 8 . Each square face of the Fe 8
cube is capped by a sulfur atom. (One terminal I per Fe is not FIGURE 39 Two common cores for octahedral metal halide clus-
shown.) ters of the general formulas (a) [M 6 X 12 ] 2+ and (b) [M 6 X 8 ] 4+ .