Carbon bridgehead, inverted bond

The bond between the bridgehead carbons of bicyclo[1.1.0]butanes can be either normal or inverted [35]. The normal bond is stabilized by the high s-character (Scheme 16a). In this case, the hybrids on the bridgeheads Y have low s-character for the bridging Y-X bonds. Electronegative atoms X or substituents R at Y stabilize... 

The propellanes are a class of compounds with three condensed rings, either three- or four-membered, sharing a bridgehead bond. In two [1.1.1] propellane derivatives studied by Seiler et al. (1988), no peaks were observed in the deformation density of the central bridgehead bonds of lengths 1.58 A, but peaks at the apex of the inverted (i.e., pyramidal) bridgehead carbon atoms are in agreement wih electrophilic attack at these positions. 

The most remarkable structural feature of [l.l.l]propellane (3) is the geometry at the bridgehead carbons. Since all four bonds point in the same direction, this might best be described as having an inverted tetrahedral geometry. 

Another feature which is not immediately recognized is that the bridgehead carbons acquire an inverted geometry, namely all four bonds of each carbon are contained in a single hemisphere. 

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