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Metal-Arene Bonding
Example question How does benzene bond to a
transition metal?
Just as we saw with ethylene, butadiene and
cyclobutadiene in Lecture 1, if we construct an
energy level diagram for the molecular orbitals
of C6H6, then the way this ligand binds to a
metal can be examined by considering which metal
orbitals have the appropriate symmetry and
spatial overlap to interact. Filling the MO
diagram with 6 electrons will also allow us to
determine which of the MOs are used for s- and p-
bonding, and which (if any) may be available for
backbonding.
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Benzene molecular orbitals
?6
Energy
?4
?5
?2
?3
?1
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Overlap with metal s, p and d orbitals
Remember that the z-axis is formally chosen to
contain the principal axis (in this case the C6
rotational axis perpendicular to the ring).
z
?1
Can overlap with s, pz and dz2 orbitals
x
y
?2
Can overlap with px and dxz orbitals
?3
Can overlap with py and dyz orbitals
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Overlap with metal s, p and d orbitals
z
?4
Can overlap with the dx2-y2 orbital
x
y
?5
Can overlap with the dxy orbital
?6
Unable to overlap with any metal orbital
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Further problems
By analogy to the MO diagram for ferrocene, you
should now be able to construct a similar energy
level diagram for (h6-C6H6)2Cr. As with ferrocene
you may assume that the dz2 orbital actually
overlaps quite poorly with the benzene pz
orbitals and therefore remains non-bonding.