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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).

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The conformational stabilities of the a-and b-substituted enamines and vinyl ethers were predicted by orbital phase theory and con®rmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital n Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the p and p* orbitals of the C¸C bond, and the r C-H or r* C-X orbitals on the substituent CH 2 X. The cyclic -n Y -p-r C-H -p*-interaction is favored by the orbital phase continuity in the a-substituted molecules, while the cyclic -n Y -p-r* C-X -p*-interaction is favored in the b-substituted molecules. The most stable conformation was then predicted to be synperiplanar or (pseudo)equatorial in the a-substituted molecules and anticlinical or (pseudo)axial in the b-substituted molecules.