Odd aromatic Si 4 ring stabilized by VV bond passing through it: May π‐bonding form without σ‐bonding as precondition?

This work theoretically presents a type of inverse sandwiches [VCl] 2 [Si 4 R 4 ] (RH, CH 3 , SiH 3 ), of which VV, VSi, and SiSi bonding modes are very odd. [VCl] 2 [Si 4 R 4 ] has the tetragonal dipyramid structure of two V atoms bridged by Si 4 ring. The eight VSi edges mainly consist of four 3c‐2e VSiV and two 4c‐2e VSiVSi bridge bonds. A VV bond passes through the square aromatic Si 4 ring. The oddest phenomenon is that no apparent SiSi σ‐bonding is found in the 4c‐6e aromatic Si 4 ring, indicating that π‐bonding may form without σ‐bonding as precondition. The orbital and bond order analyses prove the phenomenon. The odd aromatic π system may be stabilized by bridge bonds and the unique VV bond passing through the center ligand, because such phenomena are not available for [TiCl] 2 [Si 4 R 4 ]. Since π‐bondings are not so stronger than σ‐bondings, it is easy to perform interconversion between the aromatic/non‐cyclic 4c‐6e π systems of [VCl] 2 [Si 4 R 4 ].