In beforehand asking what the most important angle subtended at four-coordinate carbon is perhaps, I famous that because the angle will increase past 180°, the carbon turns into inverted, or hemispherical (all 4 ligands in a single hemisphere). So what does a seek for this case reveal within the CSD? The question might be formulated as under, by which the gap from the centroid of the 4 ligands to the central carbon is specified to be in e.g. the vary 0.8 to 1.1Å. For tetrahedral carbon surrounded by 4 carbon ligands, the worth can be near zero, so any worth bigger than say 0.8Å is value inspecting.

Most of the 101 hits are false positives for inverted carbon (by inspection), however 5 change into propellanes and eight comprise the weird motif proven under:

Right here I give one instance of every. SADHUA[1] is a crystalline [1.1.1]propellane by which the “central” bond size is a traditional wanting 1.558Å. The truth is there may be constructive (experimental) distinction electron density on each “exo” ends of this bond and adverse distinction density within the “endo” area, suggesting the bond is certainly uncommon (FAIR DOI: 10.14469/hpc/11159).

One instance of the opposite motif is SEWZID[2], the place the 4 ligands to the inverted carbon comprise two C-C bonds and two obvious C-Fe bonds of size 2.04Å. A typical C-Fe bond size is within the area 1.8Å, so these are longish C-Fe bonds. Certainly, their Wiberg bond orders emerge as ~0.3, so they might not usually rely as a “bond”. Nonetheless, they’re listed as such within the CSD! This highlights an attention-grabbing side of assemble a searchable crystal construction database. You need to decide on whether or not any pair of atoms is “bonded” or not. And the choice for bonds with orders <1 might be significantly troublesome, particularly if calculations of those properties should not a part of your task toolkit.

So we’d conclude that inverted or hemispherical four-coordinate carbon is a uncommon beast; all of the extra stunning that the very best recognized examples, the [1.1.1]-propellanes are so steady! Other than the metallocarbons, one among which is illustrated above, are there any others?