Aromaticity and hyperconjugation are two fundamental concepts in organic chemistry. By combination of the two concepts together, the resulting hyperconjugative aromaticity has attracted considerable attention from both theoretical and computational chemists. However, previous studies are mainly focused on the main group chemistry. For the hyperconjugative aromaticity in the transition metal chemistry, the studies are limited to groups 10 and 11. Here, we demonstrate that hyperconjugative aromaticity can be achieved in 2H-pyrrolium and cyclopentadiene containing group 9 transition metal substituents via density functional theory calculations. More importantly, further studies reveal that the metal–metal bonding interaction between two substituents could reduce hyperconjugative aromaticity, whereas the bridged carbonyl ligands will enhance aromaticity due to the significantly elevated HOMO orbital of the CR₂ fragment. All these findings not only extend the scope of the concept of hyperconjugative aromaticity but also are helpful to develop the chemistry of metalla-aromatics.

中文翻译：

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在2H-吡咯和环戊二烯中含有9族过渡金属取代基的超共轭芳烃的探测：桥连的羰基配体可以增强芳烃的芳香性

芳香和超共轭是有机化学中的两个基本概念。通过将这两个概念结合在一起，由此产生的超共轭芳族结构吸引了理论和计算化学家的极大关注。但是，以前的研究主要集中在主体化学上。对于过渡金属化学中的高共轭芳香性，研究仅限于第10组和第11组。在这里，我们证明了在2 H-吡咯和含环戊二烯的第9组过渡金属取代基上可实现高共轭芳香性密度泛函理论计算。更重要的是，进一步的研究表明，两个取代基之间的金属-金属键相互作用可能会降低超共轭芳族性，而桥接的羰基配体会由于CR ₂片段的HOMO轨道显着升高而增强芳族性。所有这些发现不仅扩展了超共轭芳族概念的范围，而且有助于发展金属芳香族化合物的化学。