Inverted bond and its effect on the strains of [1.1.1]propellane frameworks

The delocalization of a 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 the 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 la(M=C) > Ib (M=Si) > Ic (M=Ge) > 1d (M=Sn).