1. How does benzene bond to a transition metal?
Metal-Arene Bonding
Imperial College
London
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.

2. Benzene molecular orbitals
Imperial College
London 6
Energy 4 5 2 3 1

3. Overlap with metal s, p and d orbitals
Imperial College
London
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 x
Can overlap with s, pz and dz2 orbitals y 2
Can overlap with px and dxz orbitals 3
Can overlap with py and dyz orbitals

4. Overlap with metal s, p and d orbitals
Imperial College
London z 4 x
Can overlap with the dx2-y2 orbital y 5
Can overlap with the dxy orbital 6
Unable to overlap with any metal orbital

5. therefore remains non-bonding.
Further problems
Imperial College
London
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.