Metal-Arene Bonding
Example question:
How does benzene bond to a transition metal?
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Just as we saw with ethylene, butadiene and cyclobutadiene in Lecture 1, if we construct an energy level diagram for the molecular orbitals of C
6
H
6
, 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.

Benzene molecular orbitals

4

2

6

1

5

3
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Overlap with metal s, p and d orbitals
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Remember that the z-axis is formally chosen to contain the principal axis (in this case the C
6 rotational axis perpendicular to the ring).
z x

1 Can overlap with s , p z and d z 2 orbitals y

2 Can overlap with p x and d xz orbitals

3 Can overlap with p y and d yz orbitals

Overlap with metal s, p and d orbitals y z x

4

5

6
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Can overlap with the d x 2 -y 2 orbital
Can overlap with the d xy orbital
Unable to overlap with any metal orbital

Further problems
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By analogy to the MO diagram for ferrocene, you should now be able to construct a similar energy level diagram for ( h
6 -C
6
H
6
)
2
Cr. As with ferrocene you may assume that the d z 2 orbital actually overlaps quite poorly with the benzene p z therefore remains non-bonding.
orbitals and