Finally, we sought more insight as to why the C1•••C3 contact is so short in the structures included in this study, especially in the instance of 2. Broadly, the tendency for s-electron withdrawing groups to decrease the C1•••C3 distance in BCP rings has been established in the literature and is often attributed to some type of through-space interaction involving the linear arrangement of atoms about the C1-C3 axis (e.g., both an attractive "percaudal" interaction and a decrease in e --e -repulsion between rear orbital lobes contribute to this effect). It has also come to light that a form of hyperconjugation involving the "wing" C-C bonds may contribute significantly to stabilization (e.g., in studies pertaining to the bicyclo [1.1.1]pent-1-yl cation, among others). To study this systematically, we performed geometry optimization and natural bond orbital (NBO) secondorder perturbation analysis of SF5-BCP-Cl (2) at wB97XD/cc-pVQZ, as well as BCP (20), SF5-BCP-H (21), and Cl-BCP-H (22) for comparisons (Figure , top left). The trend in computed C1•••C3 distances (as well as qC1-C2-C3 angles) is as expected, following the increase in electronwithdrawing effect: 2 > 22 ~ 21 > 20. The Wiberg bond index between C1 and C3 also seems to follow a similar trend, increasing from 0.035 in 20 to 0.085 in 2 (while this trend is noteworthy, these values are too small to argue incipient bond formation). Additionally, by NBO analysis, the wing C-C bonds contribute a far greater two-electron stabilization energy via a side-on interaction than donor-acceptor interactions in the linear array (Figure , top left). While computed NBO energies should be carefully considered as only an "upper bound" for actual stabilization energies due to overestimation of electron delocalization effects, 32 the computed wing stabilization energies are over an order of magnitude larger than the percaudal interaction energies in each case. Moreover, the stabilization energies increase when wing C-C bonds are situated across the ring from electron-withdrawing substituents (21, 22, and 2), consistent with increased orbital overlap. Representative depictions of these key donor-acceptor interactions in 2, resembling an agostic-like donation from the wing C-C s-bond to the s*C-EWG orbital, can be found in Figure . Intuitively, a simplified resonance-like depiction of this type of wing interaction (Figure , top right) accounts for more "bonding character" between C1 and C3, and it also suggests there should be a lengthening in the C3-C2 bond opposite the EWG relative to the C2-C2 bond. For experimental support, we turned to the X-ray structure of compound 3. Here, we see a break in symmetry in both BCP rings and elongation of all wing bonds opposite the EWG's versus the adjacent ones (i.e., d(C3-C2) and d(C6-C7) ~1.56 Å vs. d(C1-C2) and d(C8-C7) ~1.54 Å). While the depiction also suggests elongation of the C-EWG bond, this is complicated for the C-Cl bond, as the stabilization energy derived from lone-pair donation into wing s*C-C orbitals is not negligible, and this would conceptually manifest in shortening of the bond (i.e., effects may cancel each other out). However, in the case of the SF5 group (devoid of a lone pair), the predicted structural manifestation of this interaction is clearer, as the C-S bond in 3 is longer than in 2 by ~1.5% (Figure ).