After much effort, we also managed to grow single crystals of 2 suitable for X-ray diffraction to examine the SF5-BCP motif in the solid state at 100 K (Figure , bottom). The most striking feature is the exceptionally short transannular C1•••C3 distance of 1.789( ) Å (libration corrected value: 1.800 Å). It is notably within error of the transannular C1•••C3 distance reported for Adcock and co-workers' pyridinium-substituted bicyclopentane -1.80(2) Å -which they claimed at the time was, "not only the shortest nonbonding contact for bicyclo [1.1.1]pentanes but also for any known organic compound." To put it in perspective, the transannular distance in 2 is approaching that of one of the longest C-C bonds reported (i.e., 1.780(7) Å). To put the structural features of the linear array of atoms about the C1-C3 axis of 2 into context, we obtained SC-XRD data for SF5-Ph-Cl (4). This allowed us to assess the consequences of Ph -> BCP isosteric replacement in the presence of an SF5 group. While maintaining a relatively linear central axis, the C1•••C3 transannular distance in 2 is attenuated by ~35% relative to the C1•••C4 distance in 4 (2.745(1) Å). The C-S bond in 2 is marginally shorter than in 4 (1.779(4) Å 21 vs. 1.7952( ) Å, respectively), while the C-Cl bond in 2 is slightly longer than in 4 (1.759(5) Å 21 vs. 1.7302(6) Å, respectively). The equatorial fluorine (Feq) atoms of the SF5 group on both 2 and 4 sport average S-Feq distances of ~1.57-1.59 Å, and qC3-S-Feq for both 2 and 4 deviates only slightly from perpendicularity on average (i.e., ~92.3º vs. ~92.6º). We also noted that the axial S-Fax bond distance in 2 at 1.579(4) Å 21 is within range of 4 (1.5869(5) Å).