How the bonding mode in cyclopentadienylaluminum complexes influences the Cp- ligand aromaticity

The substantial influence of floating Cp-Al bond-modes from η 5 (π) to η 1 (σ) on the Cp ligand's aromaticity was discussed basing on the crystal data of organoaluminum complexes and DFT calculations (B3LYP/6-311G**) performed for model compounds. The aromaticity of cyclopentadienyl moiety was characterized by the HOMA index. The spread of HOMA values from 0.8 to -0.5 indicated the electronic structure of the cyclopentadienyl ligand bonded to aluminum center can be regarded as an intermediate between the highly aromatic, uncomplexed cyclopentadienyl anion Cp - and the antiaromatic structure of cyclopentadiene CpH.