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I've heard that, even though according to Molecular Orbital Theory there is no chance of having nobel gases bonded to each other, it is not totally impossible. For example, under extreme conditions, Ar2 can be synthesised.

So I am wondering whether a carbon can make 5 bonds if the required conditions are provided.

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Here it is a nice example with TMS of a carbon covalently bound to 5 other atoms: – user2411 Oct 3 '13 at 3:31
There are informations on this topic in question about hyperlithiation. – Mithoron Dec 17 '14 at 19:30

4 Answers 4

up vote 18 down vote accepted

Carbon cannot have more then 4 double-electron bonds in reasonable conditions. However, in can form a bond with 5 or 6 atoms, like $Fe_6C$ fragment, where iron atoms form octahedron around the carbon atom. However, the sum of orders of 6 C-Fe bonds will be still 4.

The situation is different if we consider exited states. Indeed, it is possible for hight excited state of carbon to be able to form 5 or 6 bonds. The resulting structure, however, will die quickly.

Noble gases are able to form molecules in excited states, or, to be precise, their molecules are stable only in excited state. This kind of molecules is known as excimer (excited dimer).

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Carbon can make 5 bonds on methanium, $\ce{CH5+}$. It is a kind of carbonium ion and superacid. Methanium can be produced in the laboratory as a dilute and low-temperature gas.

Reaction: $$\ce{CH4 + H+ -> CH5+}$$

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I'm afraid that there are three "normal" and one 3-center-2-electron bond in methanium - 4 total – Mithoron Feb 1 at 1:33
@Mithoron I would consider the 3-center bond to involve 2 bonds to the central carbon atom. Do you see it differently? – ron May 17 at 16:12

The hypervalent carbon do exist. Look at this article: Kin-ya Akiba, et. al., J. Am. Chem. Soc., 2005, 127 (16), 5893–5901.

enter image description here

X-ray measurements confirmed the 10-C-5 structure.

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I believe the term "hypervalent carbon" is especially unfortunate for this compound (or class of compounds). They even argue with MO theory and just call it "pentacoordinate carbon" later. Nevertheless, this publication is of course a solid (pun intended) example, +1. – Martin - マーチン Apr 30 at 5:45

There are stable crystal structures where one could notionally see carbon atoms with more than 4 'bonds'. One example that comes to mind is a carborane unit; e.g. see

For example. But of course, such 'bonds' are a notional thing really and don't reflect the true nature of the underlying electronic structure.

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Your comment on notional bonds can be said of any bond unless you use a theory such as Natural Bond Order analysis, which does not necessarily contradict the penta-bonded carborane. – Deathbreath Jan 29 '13 at 19:40

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