Why Do Two Π-Electron Four-Membered Hückel Rings Pucker?

Notwithstanding their two (i.e., 4n + 2) π electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C(4)F(4)(2+)) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH(3) in C(4)(SiH(3))(4)(2+)) enhance hyperconjugation and increase the ring inversion barriers substantially.