In beforehand asking what the biggest angle subtended at four-coordinate carbon could be, 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 will be formulated as under, by which the space 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 could be near zero, so any worth bigger than say 0.8Å is price inspecting.
Most of the 101 hits are false positives for inverted carbon (by inspection), however 5 grow to be propellanes and eight include the bizarre motif proven under:
Right here I give one instance of every. SADHUA is a crystalline [1.1.1]propellane by which the “central” bond size is a standard wanting 1.558Å. In truth there may be optimistic (experimental) distinction electron density on each “exo” ends of this bond and unfavourable 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, 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’d not usually depend as a “bond”. Nonetheless, they’re listed as such within the CSD! This highlights an attention-grabbing facet of learn how to assemble a searchable crystal construction database. You must decide on whether or not any pair of atoms is “bonded” or not. And the choice for bonds with orders <1 will be significantly troublesome, particularly if calculations of those properties usually are not a part of your project toolkit.
So we would conclude that inverted or hemispherical four-coordinate carbon is a uncommon beast; all of the extra stunning that one of the best recognized examples, the [1.1.1]-propellanes are so steady! Other than the metallocarbons, one in all which is illustrated above, are there any others?