Abstract Inverse carbon‐free sandwich structures with formula E 2 P 4 (E=Al, Ga, In, Tl) have been proposed as a promising new target in main‐group chemistry. Our computational exploration of their corresponding potential‐energy surfaces at the S12h/TZ2P level shows that indeed stable carbon‐free inverse‐sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B 2 P 4 does not form the D 4 h ‐symmetric inverse‐sandwich structure, but instead prefers a D 2 d structure of two perpendicular BP 2 units with the formation of a double BB bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D 4 h inverse‐sandwich structure. The preference for the D 2 d isomer for B 2 P 4 and D 4 h for their heavier analogues has been rationalized in terms of an isomerization‐energy decomposition analysis, and further corroborated by determination of aromaticity of these species.
