Well, another class of shapely compounds seems poised to fall. Enter Pyramidanes*:
|Source: JACS | Lee, Sekiguchi|
The crew provides an in-depth discussion of frontier MO theory**, bond orders (fractional all around,
from 0.4-0.7), and NMR properties, concluding that there's a decent structural contribution from the ionic resonance form (see below).
The authors even suggest that the "cyclobutadiene" moiety - the pyramid "base" - can be stuck onto other metals; stable pyramidanes might work as strained-ring storage capsules.
Now I'm excited for the all-carbon version, which (theory predicts) has a lone pair at the apex...crazy!
*For the absurdly technical or IUPAC-minded, the authors offer alternative names such as tetracyclo-[188.8.131.52.0] pentane, or (my favorite) [184.108.40.206]fenestrane
**N.B. I'm no p-chemist, so someone else can go through the nitty-gritty there...