# Why does a diatomic carbon molecule not exist? [closed]

Many p-block elements which are the first in their respective groups show $pπ-pπ$ multiple bonding. Why doesn't carbon do so?

• What's wrong with graphite, diamond, fullerenes, and several other carbon allotropes? – Nicolau Saker Neto May 10 '15 at 17:03
• ...or all of organic chemistry. – ron May 10 '15 at 17:05
• but those are allotropes of carbon. – Anoneemus May 10 '15 at 17:06
• @shashikant So? ... They are carbons bonded to other carbons! BTW look at this : {en.wikipedia.org/wiki/Diatomic_carbon} – NeilRoy May 10 '15 at 17:11
• @shashikant I believe you are talking about carbon-carbon π bonding. Is it? – Rajat Jain May 10 '15 at 17:16

Okay, I believe that you mean to ask why a $C_2$ molecule does not exist? Well, it does exist. So, when we use molecular orbital theory to calculate the bond order of $C_2$, which is

$$B.O.=\frac{\mathrm{(No.~of~electrons~in~bonding~orbitals) - (No.~of~electrons~in~ antibonding~orbitals)}}{2}$$

We have 4 electrons in bonding and 0 in antibonding for $C_2$So putting in formula-$B.O.=(4-0)/2=2$As the bond order is 2, the molecule cannot exist.

But when a more rigorous approach to MOT is applied, it shows that both the bonds present in the molecule must be π bonds and no sigma bonds must be present. This according to me is not possible.

While Wikipedia also confirms that there is a speculation regarding the structure. It says -

A recent paper by S. Shaik reports that a quadruple bond exists in $C_2$

And hence though the actual molecule has been found, its chemistry is still being studied.

• Actually, the electrons not in the bonding orbitals are in non-bonding orbitals, not anti-bonding orbitals. – LDC3 May 10 '15 at 17:27
• $\ce{C2}$ does exist, see here – ron May 10 '15 at 17:27
• @Rajat Jain You should edit it again, now it's self- contradictory – Mithoron May 10 '15 at 19:09
• In support of ron's comment, we actually have already a question about it on the network: chemistry.stackexchange.com/q/594/4945 – Martin - マーチン May 22 '15 at 9:16
• Diatomic carbon does not have a quadruple bond but a double bond. It only exists in the gas phase. – Ali Caglayan Jun 10 '15 at 14:57