Predicting the Hyperconjugative Aromaticity in Cyclopentadiene Containing Group 8 Transition Metal Substituents

Hyperconjugative aromaticity, an integration of two chemical concepts, aromaticity and hyperconjugation, was first proposed in 1939. Recently, breaking through the main group elements, the hyperconjugative aromaticity has been successfully extended to the transition metal system, including groups 7, 9, 10, and 11 organometallic substituents. Here, we demonstrate that the missing group 8 transition metal substituents also possess a powerful ability to induce hyperconjugative aromaticity in cyclopentadiene via density functional theory calculations. It is found that metal–metal bonding will distinctly reduce the hyperconjugative aromaticity. Unexpectedly, the combination of a metal–metal double bond with a bridged carbonyl can significantly restore the aromaticity. Our findings expand the scope of both hyperconjugative aromaticity and metallaaromatic chemistry.