Bonding Analysis in [1.1.1]Propellane and [1.1.1]Bicyclopentane Using Orbital Forces. The Myth of the “Inverted Bond”

The properties of the “inverted bond” in [1.1.1]propellane are investigated by two methods. Firstly we study H3C-C models of C-C bonds with frozen HCC angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane and [1.1.1]bicyclopentane are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate in-situ bond energies. Triplet state of propellane and cationic states of propellane and bicyclopentane are also considered to comfort the bonding/antibonding character of MOs in the parent molecules. Both approaches shows an essentially nonbonding or slightly repulsive character of the sigma central CC interaction in propellane: the so-called ‘inverted’ bond, as resulting from a sigma overlap of the two s-p hybrids by their smaller lobes, appears devoid of physical basis. The bonding of central CC in propellane is thus only due to pi-type MOs (also called ‘banana’ MOs or ‘bridge’ MOs) and its total energy is evaluated to ca. 60 kcal/mol. In bicyclopentane, despite a strong sigma-type repulsion, a weak bonding (20 kcal/mol) exists between both central CC, also due to pi-type interactions, though no formal bond is present