In previously asking what the largest angle subtended at four-coordinate carbon might be, I noted that as the angle increases beyond 180°, the carbon becomes inverted, or hemispherical (all four ligands in one hemisphere). So what does a search for this situation reveal in the CSD?
AbstractThe molecular structure and electron‐density distribution of two [1.1.1]propellane derivatives have been determined from accurate single‐crystal X‐ray diffraction measurements at 81 K. The crystals of these highly reactive compounds (both are liquid at room temperature) were grown directly on the X‐ray diffractometer at ca. 208 and 228 K, respectively. Both compounds crystallize in the space group P21/c with one molecule in the asymmetric unit. The symmetry of the propellane C‐atom skeleton is close to D3h for both molecules. The distances between the two bridgehead C‐atoms are 1.587 and 1.585 Å, and the mean lengths of the propellane side bonds are 1.525 and 1.528 Å, respectively. The deformation density peaks of the propellane side bonds (ca. 0.25 e/Å3) lie somewhat outside the internuclear connection lines and so correspond to ‘bent bonds’ as expected. On the other hand, the difference electron density between the bridgehead nuclei is slightly negative in both molecules. An interesting feature, observed in these difference maps, is presence of a diffuse, positive difference density at each inverted C‐atom, outside the bridgehead bond, a probable site of electrophilic attack.