An Isolable Radical Anion Featuring a 2‐Center‐3‐Electron π‐Bond without a Clearly Defined σ‐Bond

Abstract Featuring an extra electron in the π* antibonding orbital, species with a 2‐center‐3‐electron (2c3e) π bond without an underlying σ bond are scarcely known. Herein, we report the synthesis, isolation and characterization of a radical anion salt [K(18‐C‐6)] + {[(HCNDipp) 2 Si] 2 P 2 }⋅ − (i.e. [K(18‐C‐6)] + 3 ⋅ − ) (18‐C‐6=18‐crown‐6, Dipp=2,6‐diisopropylphenyl), in which 3 ⋅ − features a perfectly planar Si 2 P 2 four‐membered ring. This species represents the first example of a Si‐ and P‐containing analog of a bicyclo[1.1.0]butane radical anion. The unusual bonding motif of 3 ⋅ − was thoroughly investigated via X‐ray diffraction crystallography, electron paramagnetic resonance spectroscopy (EPR), and calculations by density functional theory (DFT), which collectively unveiled the existence of a 2c3e π bond between the bridgehead P atoms and no clearly defined supporting P−P σ bond.