The origin of the inverted tetrahedral configurations of the bridgehead carbons in bicyclo[1.1.0]butane has been investigated in a theoretical manner. The delocalization of σ-electrons between the central C 1 -C 3 bond and the side C 1 -C 2 bonds was found to be bonding to relax the angle strains, while the delocalization between the geminal C-C bonds is antibonding in propane and cyclopropane, etc. The bonding property of the geminal delocalization was shown to be related to the inversion of the configurations. Inverted and normal tetrahedral bridgehead configurations in the bicyclo[1.1.0]butane frameworks were designed by controlling the geminal delocalization and confirmed by ab initio molecular orbital calculations.

The delocalization of σ electrons from a normal bond to a geminal “inverted” bond was shown to be strongly bonding due to the orbital phase property. The bonding property of the geminal delocalization was predicted to increase with bond inversion. The central bond between the bridgehead atoms in the [1.1.1]propellane frameworks (1) is inverted to relax the ring strains as the row of the elements is high in the periodical table. The strain decreases in the order of 1a (M = C) > 1b (M = Si) > 1c (M = Ge) > 1d (M = Sn).