Question

Are there any compounds with pentavalent carbon?

Answer 1

Scientists first prepared methanium ion in 1950.

Explanation:

Methanium ion

Fluoroantimonic acid `"HSbF"_6` reacts with methane to form methanium ion, `"CH"_5^"+"`.

It is a fluxional ion, in which the `"C-H"` bonds are continually breaking and re-forming even at 4 K.

In the animation above, the black ball in the middle is the carbon atom, and the red and white ones are hydrogen atoms. The blue clouds represent the electron pairs.

A theoretical pentacoordinate carbon

Theoretical chemists have predicted that `"CBe"_5^(4-)` would have a planar pentacoordinate carbon and might even be stable in the gas phase.

The bonds are not ordinary σ bonds. Instead, they involve various σ and π orbitals involving the whole molecule.

`"CAl"_4"Be"` and `"CAl"_3"Be"_2^-` are also theoretical possibilities.

Heptacoordinate carbon?

Another theoretical possibility is `"CTi"_7^(2+)`, with a `"C"` atom at the centre of a pentagonal bipyramid — a heptacoordinate `"C"` atom!