A neutral σ⁰π² carbene enabling hydrogen activation via a σ-face pathway

Ground-state σ⁰π² carbenes are among the least explored and least understood classes of low-valent carbon. Known examples of isolable σ⁰π² carbenes remain largely limited to cationic compounds. Here we report the synthesis of a neutral σ⁰π² carbene, namely a rhodadiphosphinocarbene featuring a planar RhP₂C ring. In sharp contrast to small-molecule activation by conventional σ²π⁰ carbenes, which typically proceeds via nucleophilicity-driven, π-face, non-least-motion trajectories mandated by orbital-symmetry constraints, this neutral σ⁰π² carbene cleaves H₂ under ambient conditions via a σ-face pathway. Computations reveal an electrophilicity-driven early transition state characterized by minimal reorganization of the carbene framework and a concerted yet asynchronous H-H scission to form two C-H bonds. These findings show that a stable carbene can enable small-molecule activation through a σ-face pathway and deepen our understanding of electronic structure-reactivity correlations in low-valent carbon chemistry.