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 will be formulated as beneath, wherein 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 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 transform propellanes and eight include the weird motif proven beneath:
Right here I give one instance of every. SADHUA[1] is a crystalline [1.1.1]propellane wherein the “central” bond size is a traditional trying 1.558Å. In reality 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[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’d not usually depend as a “bond”. Nonetheless, they’re listed as such within the CSD! This highlights an fascinating side of tips on how to 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 will be significantly troublesome, particularly if calculations of those properties are usually not a part of your task toolkit.
So we would conclude that inverted or hemispherical four-coordinate carbon is a uncommon beast; all of the extra stunning that the perfect identified examples, the [1.1.1]-propellanes are so steady! Aside from the metallocarbons, considered one of which is illustrated above, are there any others?
