Benzene Ligands

Benzene (η⁶-C₆H₆)

Electron Contribution: 6 electrons (neutral ligand).

Hapticity: η⁶ (hexahapto coordination).

Key Features:

• Aromatic π-system used in organometallic complexes.
• Example: [Cr(η⁶-C₆H₆)(CO)₃] or ruthenium (II) biphenyl TsDPEN

Bonding and Coordination:

Hapticity:

• benzene ligands typically coordinate in a η⁶ fashion, meaning that all six of the carbon atoms in the ring participate in binding to metal center through delocalized π-electrons.
• Occasionally, partial coordinations (η⁴ or η²) complexes can form but these are rare. 

Nature of Bonding:

• Metal to ligand π-backbonding: electrons from filled metal d-orbital can be donated into the π^* orbitals of the benzene ligand. 
• Metal to ligand π-donation: electrons from the aromatic π-system are donated into empty metal orbitals. 
• These two interactions result in synergic bonding, similar to carbonyls or cyclopenadienyls, althrough weaker due to benzenes reluctance to distort it's aromaticity. 

Structural Geometry:

• Benzene rings can adopt the same coordination geometries as were discussed in the section for cyclopentadienyl ligands.
• Sandwich, piano-stool, and bent sandwich conformations possible.

Synthesis of Benzene Complexes:

Direct Reaction: Transition metals (especially early and mid-series) can react with benzene under reducing conditions:

• 2CrCl₃ ​+3Al+2C₆​H₆ ​--> ​[Cr(η⁶−C₆​H₆​)₂​]

Ligand Exchange: Arene ligands can displace weakly bound neutral ligands (e.g., ethers, arenes) in pre-formed complexes.

Electronic Structure and Stability:

Electron Count:

• Benzene ligands, like cyclopentadienyl ligands, contribute 6 electrons to the total electron count.

Oxidation States:

• Arene complexes often stabilize low oxidation state complexes.

Reactivity:

Ligand Substitution: 

• Arene ligands can be displaced by stronger donors (e.g. phosphines or carbonyls) but usually require heat or light. 

Nucleophilic Aromatic Substitution (NAS):

• Coordination of benzene to metal center depletes electron density which leaves the benzene susceptible to NAS reactions. 
• Note: NAS reaction occured on para position but could change depending upon the electronic properties of the R group. 
• The introduction of an oxidizer can also dearomatize the ring while selectively adding to one face.

Hydrogenation:

• Under catalytic or hydrogenating conditions, coordinated benzene rings can undergo partial or full hydrogenation.

Proposed Image: A structure of ruthenium (II) biphenyl TsDPEN complex, highlighting benzene's η⁶ coordination.  This is an asymmetric transfer hydrogenation catalyst for a range of primarily acetophenone derivatives https://doi.org/10.1016/j.jorganchem.2018.08.020