Imperial CollegeLondonMetal-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 - and - bonding, and which (if any) may be available for backbonding.

Imperial CollegeLondonBenzene molecular orbitals

Ene

rgy

1

2 3

4 5

6

Imperial CollegeLondonOverlap 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

xy

1 Can overlap with s, pz and dz2 orbitals

2 Can overlap with px and dxz orbitals

3 Can overlap with py and dyz orbitals

Imperial CollegeLondonOverlap with metal s, p and d orbitals

z

xy

4 Can overlap with the dx2-y2 orbital

5 Can overlap with the dxy orbital

6 Unable to overlap with any metal orbital

Imperial CollegeLondonFurther problems

By analogy to the MO diagram for ferrocene, you should now be able to constructa similar energy level diagram for (6-C6H6)2Cr. As with ferrocene you may assumethat the dz2

orbital actually overlaps quite poorly with the benzene pz orbitals andtherefore remains non-bonding.