Probing the Hyperconjugative Aromaticity of Cyclopentadiene and Pyrroliums Containing Group 7 Transition Metal Substituents

Aromaticity and hyperconjugation are two fundamental
concepts in
chemistry. Combining them together led to the proposal of the concept
of hyperconjugative aromaticity by Mulliken in 1939. Now, it has been
attracting considerable attention from both theoretical and experimental
chemists. Recently, the concept of hyperconjugative aromaticity has
been extended from main-group substituents to transition metal systems
including groups 9, 10, and 11 transition metal substituents. Here,
we report the hyperconjugative aromaticity in cyclopentadienes and
pyrroliums containing group 7 transition metal substituents through
density functional theory (DFT) calculations. It is found that the
metal–metal bonding interaction can significantly reduce the
aromaticity in cyclopentadienes, whereas the stronger σ-donor
ligand, bridged hydride, and carbonyls can enhance aromaticity. All
these findings expand the scope of the concept of hyperconjugative
aromaticity, enriching aromatic organometallic chemistry.