Inverted bonds

Keywords tr-Relaxation o-Relaxation Geminal interaction Inverted bond Lone pair effect Orbital phase continuity Ring strain... 

The bond between the bridgehead atoms is inverted in the [l.l.l]propellane system 59. The hybrid orbitals are directed away from each other, hi the geminal 0-0 interaction, the greater lobes of the geminal hybrid orbitals at the bridgehead atom can be in phase with each other, while the front lobes of the hybrid orbitals of the normal and inverted bonds at the 1,3-positions are in phase with each other... 

Scheme 25 Bonding geminal a-a interaction with the inverted bond in the propellane 59...

In this molecule, the pair of bridgehead atoms each has all four bonds directed to the same side as its partner atom, and the central bond length is 164.3 pm. Thus the bridgehead atoms have an inverted bond configuration and serious C-C bond strain. 

FIGURE 1. Frontier MOs in square-planar metal-dithiolene complexes formed by the in-phase (i.p.) and out-of phase (o.o.p.) combinations of C2S2 orbitals perturbed by the metal. The d metal orbitals stabilized relative to ligand orbitals lead to an inverted bonding scheme . Reprinted with permission from Reference 12. Copyright 2004 American Chemical Society... 

This Inverted Bond was Suggested to be a Charge-Shift Bond Wu, W., Gu, J., Song, J., Shaik, S., Hiberty, P. C. (2009). The inverted bond in [l.l.l]propellane is a charge-shift bond. Angewandte Chemie International Edition, 48(S), 1407-1410. 

Variable Energy Valence Photoelectron Spectroscopy—Inverted Bonding Description 696... 

The existence of the CS bond family will eventually be cmisolidated by experimental means. One such technique involves the determination of the Laplacian of various bonds from experimental densities, as already done for propeUane [58], N2O4 [73], (Mg-Mg) " cores [74], etc. [56, 57]. Thus, as we already alluded to, for [l.l.l]propellane derivatives [58], the experimental Laplacian clearly shows the existence of a classical covalent C-C bond in the wing positions vis-a-vis a CS C-C bond in the inverted bond region. Such experimental characterization for other bonds will further show the importance of this new bond family. 

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