Ring-Strain-Formation Lewis Acidity? A Pentacoordinate Silacyclobutane Comprising Exclusively Equatorial Si−C BondsDedicated to Prof. Dr. Reinhold Tacke on the occasion of his 60th birthday.

Although silacyclobutanes appear susceptible to penta- and hexacoordination of their Si atoms because of their inherent so-called ring-strain-release Lewis acidity, we succeeded in synthesizing a pentacoordinate (trigonal-bipyramidal) silacyclobutane with equatorial Si−C bonds. A related silacyclobutane with an axial−equatorial Si−C situation was found by quantum-chemical calculations to have a low energy barrier (ca. 3 kcal/mol) for its transformation into an isomer with exclusively equatorial Si−C bonds.