Switchable dual bonding nature in silabicyclo[1.1.0]butanes that exhibit bond stretch isomerism

Silicon analogues of bicyclo[1.1.0]butanes are known to exhibit a bond stretch isomerism where the bridgehead SiSi bond changes in both length and geometry. The bridgehead silicon atom of long bond (lb) isomers exhibits an unusual geometry where all the bonds are located on the same hemisphere, making the geometry of the bridgehead SiSi bond the so-called inverted σ bond in contrast with the typical σ bond geometry found in the short bond (sb) isomers. Herein, the bonding nature of bridgehead SiSi bonds was theoretically investigated in detail using valence bond theory calculation at SL-BOVB/6-31G(d) level of theory. Our calculation demonstrates that the bond type of the bridgehead bond depends on the geometry around the bridgehead atoms with those in sb and lb isomers classified as covalent (COV) and charge-shift (CS) bonds, respectively. The different bond types between two bond stretch isomer indicate that the bonding nature is switchable in one molecule. The detailed examination of the bond stretch isomerization in parent 1,3-disilabicyclo[1.1.0]butane demonstrated that the enhanced CS bond nature in the lb isomer is mainly attributed to the geometry inversion of the σ bond rather than the bond elongation. This switchable dual bonding nature provides an important platform to compare covalent and CS bond on equal footing for both theoretical and experimental investigations.