Is B2 a compound with no sigma bond but only a pi bond?

  • Thread starter Indranil
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In summary, the compound with no sigma bond only with the pi bond is not possible. The reason is that simple covalent bonds always have a sigma bond and a double/triple covalent bond has one sigma bond and 1 or 2 pi bonds (respectively). Higher spin compounds can overcome this steric constraint.
  • #1
Indranil
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Could suggest a compound with no sigma bond only with the pi bond?
 
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  • #2
Hi,

This kind of compound does not exist, as long as simple covalent bond is sigma bond and a double/triple covalent bond have one sigma bond and 1 or 2 pi bonds (respectively).
 
  • #3
When you think about it, any atom with, say, p orbitals available to form pi bonds would also be able to form sigma bonds with them; and that sigma would be more energetically favoured. So we would have to rely on steric constraints preventing sigma bond formation. It is difficult to imagine a scenario where that would be the case. Can you think of one?
 
  • #4
This depends on the precise definition of a sigma bond. I'm not sure there is one. If you require cylindrical symmetry of the bond (as in the Wikipedia article), then you have to contend with the fact that C2H2 (or any alkyne) has a cylindrically symmetric pi electron density. On the other hand, it's unclear whether you can call the interaction between Fe and Cp in ferrocene a sigma interaction, even though the bonding is clearly covalent and quite strong.

I suppose you could make the argument that a sigma bond is one where the maximum electron density lies on the line connecting the two atom centers (this would exclude the pi electron sheath in acetylene, whose wavefunction has a node along this line). I can't think of any attractive atom-atom interaction where this criterion would not be present. Maybe in side-bonded H2 or ethylene η2 transition metal complexes? The bond here is formed between a filled metal d-orbital and the empty antibonding orbital (σ* for H2 and π* for C2H4), along with a "backbond" between the filled H-H or C=C bonding orbitals and an empty metal d-orbital. (I use quotes because I'm not sure that the metalheads consider this a true backbond.) It would require an eelctronic structure calculation to figure out the maximum electron density of this bonding configuration. Maybe I'll give it a shot if I have time.
 
  • #5
Yes, the back-bonding in complexes of ethylene with metal ions would certainly be candidates for pi bonds - though the forward donation part of the bonding could be construed as 2 sigma bonds. This is territory where things are not so clean-cut!
 
  • #6
Indranil said:
Could suggest a compound with no sigma bond only with the pi bond?
B2
 
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  • #7
DrDu said:
B2

B2 is a good call! It does indeed seem to be a case where 2 "half pi" bonds are stronger that a single sigma bond. However B2 is paramagnetic so high spin could play a part in that.
 
  • #8
Jason2 said:
B2 is a good call! It does indeed seem to be a case where 2 "half pi" bonds are stronger that a single sigma bond. However B2 is paramagnetic so high spin could play a part in that.
Another effect which contributes here is that the 2pz orbitals and the resulting sigma bond get pushed up by the 2s orbitals on the neighbouring atom. On the other hand, the 2s orbitals get lowered in energy. Probably a careful calculation of the bond orders would reveal that there is a net sigma bonding contribution though much smaller than in typical sigma bound compounds.
This mechanism becomes unimportant in later compound of the series, as the s-p energy difference increases when going from B to C, N, O, F. E.g. in N2, the sigma bond formed from the pz orbitals is lower than the pi bond orbitals.
 

1. What is a compound with no sigma bond?

A compound with no sigma bond is a chemical compound in which all bonds between atoms involve only pi bonds, and there are no single bonds present. This type of compound is commonly referred to as an "allene" or a "cumulene".

2. How is a compound with no sigma bond different from other compounds?

A compound with no sigma bond is different from other compounds because it has a unique molecular structure with alternating single and double bonds. This results in a linear or bent shape, rather than the typical tetrahedral or trigonal planar shapes of most compounds.

3. What are some examples of compounds with no sigma bond?

Some common examples of compounds with no sigma bond include allenes such as propadiene, and cumulenes such as butatriene. These compounds are commonly used in organic synthesis and have important applications in materials science and pharmaceuticals.

4. How are compounds with no sigma bond formed?

Compounds with no sigma bond are formed by the overlapping of p orbitals between atoms, rather than the overlapping of s and p orbitals which forms a sigma bond. This type of bonding is common in compounds with multiple double bonds and can also occur in strained molecules.

5. What are the properties of compounds with no sigma bond?

Compounds with no sigma bond have unique physical and chemical properties. They are often highly reactive due to the presence of multiple double bonds, and their linear or bent shape can affect their polarity and solubility. These compounds also exhibit interesting optical properties and are used in the production of OLEDs and other electronic devices.

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