Benzene and other arenes

Key point: A consideration of the MOs of benzene leads to a picture of bonding of benzene to a metal atom that includes a significant δ back bonding interaction. If benzene is considered to have three localized double bonds, each double bond can behave as a ligand and the molecule could behave as a tridentate η6-ligand. A compound such as bis(η6-benzene) chromium (40) could then be considered to be made up of six coordinated double bonds, each donating two electrons, bound to a d6 metal atom, giving a total of 18 valence electrons for the octahedral complex. Bis(η6-benzene) chromium does exist, and is remarkably stable: it can be handled in air and sublimes with no decomposition. Although this description of the bonding is a first step towards understanding its structure, the true picture needs a deeper consideration of the molecular orbitals involved. In the molecular orbital picture of the π bonding in benzene there are three bonding and three antibonding orbitals. If we consider a single benzene molecule bonding to a single metal, and consider only the d orbitals, the strongest interaction is a σ interaction between the most strongly bonding a1 benzene MO and the d_z2 orbital of the metal atom; π bonds are possible between the two other bonding benzene MOs and the d_zx and d_yz orbitals. Back bonding from the metal atom to the benzene is possible as a δ interaction between the d_x2-y2 and d_xy orbitals and the empty antibonding e₂ orbitals of benzene (Fig. 22.8). η6-Arenes are considered to be neutral ligands that donate six electrons and are normally considered to take up three coordination sites at a metal.

Hexahapto (η⁶) arene complexes are very easy to make, often simply by dissolving a compound that has three replaceable ligands in the arene and refluxing the solution:

One commonly invoked reaction intermediate of η⁶-arene complexes is a ‘slipping’ to an η⁴ complex, which donates only four electrons to the metal, and therefore allows a substitution reaction to proceed without an initial ligand loss:

η²-Arenes are also known and are analogous to η²-alkenes; they have an important role in the activation of arenes by metal complexes.

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