Coordination compounds metal-ligand resonance interaction

Coordinated polyenes and arenes are a simple, but important, class of ir-donor ligand. Although we considered an rj -arene ligand to be a six-electron donor in Section 1.1, because it contains resonance structures with three double bonds, each of which are two-electron donors, the bonding of such ligands to the metal is more complex. These ligands are more accurately described by a combination of a- and ir-donation from the molecular orbitals of the entire ring. This combination of interactions is illustrated by the molecular orbitals of a sandwich compound, such as bis(benzene)chromium. 

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