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We revealed the origin of the structural features of a series of tetrasilacyclobutadiene analogues based on a detailed study of their electronic structure and the pseudo Jahn-Teller effect (PJTE). Starting with the D 4h  symmetry of the Si 4 R 4  system with a square four-membered silicon ring as a reference geometry, and employing ab initio calculations of energy profiles along lower-symmetry nuclear displacements in the ground and several excited states, we show that the ground-state boat-like and chair-like equilibrium configurations are produced by the PJT interaction with appropriate excited sates. For Si 4 F 4  a full two-mode  b  1g − b  2g  adiabatic potential energy surface is calculated showing explicitly the way of transformation from the unstable D 4h  geometry to the two equilibrium C 2h  configurations via the D 2h  saddle point. The PJTE origin of these structural features is confirmed also by estimates of the vibronic coupling parameters. For Si 4 R 4  with large substituents the origin of their structure is revealed by analyzing the PJT interaction between the frontier molecular orbitals. The preferred chair-like structures of Si 4 R 4  analogues with amido substituents, and heavier germanium-containing systems Ge 4 R 4  (potential precursors for semiconducting materials) are predicted.

Geometry, Electronic Structure, and Pseudo Jahn-Teller Effect in Tetrasilacyclobutadiene Analogues