Quadruple bond between two carbon atoms

[pierce15]

I was wondering if a quadruple bond between two carbon atoms would be possible. I found this on wikipedia about "dicarbon": http://en.wikipedia.org/wiki/Diatomic_carbon, which is related to this question, since what I'm describing is an isomer of dicarbon. Anyone have any ideas?

 

[DrDu]

The wikipedia article is not very good. Valence bond theory also predicts a double bond as carbon atoms have two unpaired electrons. It is not a requirement of VB theory for the molecule to obey the octet rule.
Higher bond orders may be present in electronically excited state of C2.

 

[pierce15]

So what do you think about a quadruple bond?

 

[hilbert2]

Whether bonding in dicarbon is better described as a double or quadruple bond is still being debated, see http://www.ch.imperial.ac.uk/rzepa/blog/?p=10733 .

Assigning definite integer-valued bond orders to chemical bonds is just a property of the approximative model used to describe bonding.

 

[cgk]

Whatever else one may or may not say about C2, if it is described with a single closed-shell determinant wave function (i.e., with Hartree-Fock or Kohn-Sham), one can rotate its occupied orbitals into two equivalent doubly-occupied non-standard sigma bond orbitals and two standard doubly occupied pi bond orbitals via orbital localization (Pipek-Mezey-like).

As the theoretical basis for bond order concept is closely related to this form of MO theory (which, for C2, is not entirely beyond question--it has low lying excited states), one could formally interpret this as a quadruple bond. Or as a double bond or triple bond, depending on how one wishes to interpret the non-standard sigma system.

In short, it is the perfect fighting ground for bored theorists 8).

 

[DrDu]

Another question is what a number like "bond order" really tells you.
What we observe are bond lengths and strengths and not bond orders.