Texas Carbon

A common mistake made in drawing organic compounds is exceeding the valency of a carbon atom:

Colloquially, carbon atoms with five covalent bonds are called Texas Carbons⊕
Either because the Texas star has five points or because everything is bigger in Texas.. Formally, they are known as pentavalent carbons.

It turns out that well-formed pentavalent carbons are much more frequent than an organic chemistry class may have you believe.

Super​$^5$Acids

Superacidic cationic pentavalent carbons can be formed in the presence of very strong acids. The simplest pentavalent carbon, methanium (​$\text{CH}_5^+$), can be prepared from methane in the presence of ​$\text{SbF}_5 + \text{HF}$.

Ethanium (​$\text{C}_2\text{H}_7^+$) is a protonated ethane derivative, also hosting a pentavalent carbon. Ethanium is interesting partly because it is prepared using methanium:

and partly because it easily dissociates into ethenium, which also has pentavalent carbons:

In practice, pentavalent acids are formed as intermediate species and during thermal crackingPer a DOE report, page 8..

Both B and C

Yamamotoa's and Akibab's 2004 Synthesis of Hypervalent Pentavalent Carbon and Boron Compounds describes the characteristics of larger pentavalent compounds. It appears (somewhat expectedly) that bonds to well-charged, larger-size atoms like those in Group 15 and 16 help stabilize pentavalent carbon and boron.

The bond angle of the emphasized ​$\text{O}-\text{C}-\text{O}$ is ​$\sim 160^\circ$, more than the ​$144^\circ$ expected for a star-shaped pentavalency, and likely explained by the distance of the oxygens from the carbon.

Are there star-shaped pentavalencies?

Yes! One is stable methanium, which we have already seen. Another is ​$\text{CBe}_5^{4-}$, which is predicted to be planar and possible to prepare experimentally in the gas phase. Grande-Aztatzi et al. predict the lithium cluster derivatives ​$\text{CBe}_5{Li}_n^{n-4}, n \in [1, 5]$ to also be planar. Both of these form beautiful five-point shapes.

Beyond the stars

Pentavalent carbons are one of those ideas most of us won't deal with very often. But I think unique patterns like these are worth knowing, at least for the sake of knowing.

If you're interested in hypervalencies, you may enjoy hexavalent carbons and undecavalent borons.