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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

538 Metal Particles and Cluster Compounds

FIGURE 23 Isolobal relationship between B H and ML 3 frag-
ments. Two alternative ways of viewing the hybridization are FIGURE 24 Structures of Re 4 (CO) 12 H 4 and [Re 4 (CO) 12 H 6 ] 2− .
shown. The B H group may be considered to be sp hybridized in The carbonyls of Re 4 (CO) 12 H 4 are staggered with respect to the
which case a metal d 2 p z hybrid mimics the radially oriented sp tetrahedral edges. The carbonyls of [Re 4 (CO) 12 H 6 ] eclipse the
z
3
orbital on boron. The sp hybridization of a B H group may be tetrahedral edges.
mimicked by the orbital lobes of an ML 3 unit which are directed to
three facial octahedral sites.
the polyhedral edges. Re 4 (CO) 12 H 4 is one compound in
which such carbonyl orientation is observed (Fig. 24). Al-
3
p-orbitals which remain unhybridized are oriented tan- ternatively, the sp -like vertex orbitals may be rotated
◦
gentially to the cluster. The sp hybrid points inward to 60 thereby directing the orbital lobes over the triangu-
the cluster core and can participate in significant bonding lar faces of the polyhedron. This orientation, which sug-
interactions as shown below for an octahedral cluster. The gests three center bonding interactions over the tetrahedral
unhybridized p-orbitals are oriented such that they may faces, causes the carbonyl to eclipse the polyhedral edges.
contribute to surface bonding. These “surface” orbitals al- This configuration is seen in [Re 4 (CO) 12 H 6 ] 2− (Fig. 24).
low for interactions between adjacent cluster vertices as In order to predict or rationalize a geometry of a metal
well as contributing to cluster–ligand interactions. The or- cluster by the PSEP theory, the number of cluster valence
bitals of a metal fragment can easily mimic those of the electrons must be determined. The electron donation from
2 p z hybrid of a metal can play a metallic molecular fragment [M(CO) 3 , for example] is
sp hybridized boron. A d z
the role of the radially oriented sp hybrid while d xz p x and determined as follows. Assume all nine valence orbitals
d yz p y hybrids play the role of the tangentially oriented p are used. Through the isolobal analogy three of these nine
orbitals. orbitals of the metal are used for cluster bonding. This
The sp hybridization is just one of many ways to look leaves six orbitals to serve in metal–ligand bonding or
at the bonding in the borane polyhedra. Alternatively, an nonbonding (lone pair) capacities. These six orbitals will
3
sp hybridization may be considered. The B H bond uti- accommodate 12 electrons. Therefore, the electron dona-
3
lizes one of the four orbitals in sp hybridization leaving tion to a cluster from a metallic molecular fragment is the
three orbitals to participate in cluster bonding. The or- total valence electron count at the metal center less 12
3
bitals a metal could use to imitate the sp hybrid are easily electrons. For example, Fe(CO) 3 has a 14-electron count
8
conceptualized. Imagine an octahedral ML 6 species being at the iron center [d Fe + three (2-electron) donors]. Of
stripped of three facial ligands and the resulting ML 3 unit these 14 electrons 12 are allocated to the Fe CO bonds
being incorporated into a cluster. The orbitals directed to- and nonbonding or lone pair orbitals which leaves two
ward the three vacant coordination sites are those orbitals electronstobesuppliedtothecluster.Likewise,themolec-
3
which are similar to the sp orbitals and can be used for ular fragment Co(C 5 H 5 ) contains 14 electrons at the metal
9
cluster bonding (Fig. 23). One advantage of considering center (d Co + 5 electron donor), which makes this a two-
an M(CO) 3 molecular fragment to be isolobal with an electron donor to a cluster. These two metallic molecu-
3
sp hybridized boron vertex is realized when attempting lar fragments each donate two electrons as do (BH) and
to rationalize the observed stereochemical configuration (CH ) units. Therefore, it is not surprising that these
+
3
of some metal carbonyl clusters. The sp type vertex or- isolobal fragments are interchangeable giving rise to a
bitals may be oriented along the edges of the triangular large family of cluster compounds known as metallobo-
units of a cluster. This orbital orientation, which suggests ranes and metallocarboranes.
two-electron–two-centerbonds,resultsinthecarbonyllig- Metalloboranes or metallocarboranes are compounds in
ands of the M(CO) 3 unit being staggered with respect to which one or more vertices of a parent borane/carborane