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0 25050158

DOI: https://doi.org/10.6023/A25050158

Development and Outlook of Carbenes with Inverted Electronic Configuration (σ0π2)

  • Liu Zhiqing ,
  • Liu Liu Leo
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  • Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055

Received date: 2025-05-11

  Online published: 2025-06-25

Supported by

National Natural Science Foundation of China (22350004).

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Abstract

Carbenes (R₂C:) represent a unique class of neutral divalent carbon species that formally possess six valence electrons at the central carbon atom, thereby deviating from the octet rule. These species have long occupied a central role in organic, organometallic, and materials chemistry owing to their rich electronic structures and versatile reactivity. The vast majority of isolated stable carbenes adopt a singlet ground-state electronic configuration described as σ²π⁰, wherein a lone pair occupies an in-plane σ orbital and the out-of-plane π orbital remains unoccupied. This configuration underpins the pronounced nucleophilicity observed in classical N-heterocyclic carbenes and their analogs. A limited number of carbenes with a σ¹π¹ open-shell ground state have been spectroscopically characterized, often exhibiting radical-like behavior and enhanced reactivity. In sharp contrast, carbenes with an inverted electronic configuration—σ⁰π²—remain extraordinarily rare and conceptually intriguing. These carbenes possess two electrons fully occupying the out-of-plane π orbital, leaving the in-plane σ orbital vacant. Such a configuration inverts the typical orbital occupancy pattern, potentially leading to electrophilic or ambiphilic reactivity profiles. To date, four principal design strategies have been proposed to stabilize σ⁰π² carbenes: (1) cyclic di(imino)carbenes; (2) diboryl-substituted carbenes; (3) metal-coordinated cyclic di(phosphino)carbenes; and (4) dicationic carbones. However, despite extensive conceptual exploration, only one stable and isolable σ⁰π² carbene has been structurally authenticated to date. This species features a rigid RhP₂C four-membered metallacycle, wherein the carbene carbon exhibits an unprecedented 13C NMR chemical shift below -30 ppm, representing the most upfield resonance recorded for a carbene center. This carbene demonstrates divergent reactivity patterns uncharacteristic of classical σ²π⁰ congeners. It reacts with both Lewis acids and Lewis bases to yield coordination adducts and π-complexes, respectively, and participates in bond-forming processes such as ketenimine generation from isocyanides. Its demonstrated ambiphilicity offers promise for the activation of inert small molecules, including dinitrogen (N₂)—a long-standing goal in main group chemistry. This Perspective provides a systematic overview of the emerging chemistry of σ⁰π² carbenes, critically analyzing structural parameters, stabilization frameworks, and reactivity paradigms. We further highlight the prospective roles of such carbenes in bond activation and novel ligand design, envisioning their integration into next-generation main-group and transition-metal cooperative systems.

Key words： carbene; inverted electronic configuration; synthetic chemistry

Cite this article

Liu Zhiqing , Liu Liu Leo . Development and Outlook of Carbenes with Inverted Electronic Configuration (σ0π2)[J]. Acta Chimica Sinica, 0 : 25050158 . DOI: 10.6023/A25050158

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