What are the relative strenghts of pi and sigma bonds and why ?

I know it doesnt happen this way but if you could isolate a pi and a sigma bond which would be stronger and why ( I believe the sigma to be stronger)

Reply 1

JessicaLucy09

more bond to atom makes the bond stronger & shorter

Bond Length
single bond > double > triple

Bond strength
Singe bond < double < triple

Basically addition of pi bond increases atomic orbital overlap (increasing wave function) that increases bond strength and shortens bond length

Reply 2

Macky16

Original post by JessicaLucy09

Im not asking if a compound with a pi bound has a stronger bond ...... im asking if you were to look at a molecule with both pi and sigma bonds between two atoms which of the two bonds is ,in isolation, stronger and why

Reply 3

Pigster

sigma pwns pi.

The bond lengths and enthalpies:

C—C 0.154 347
C=C 0.134 612
C∫C 0.120 838

i.e. the second (pi) bond is weaker than the first (sigma) 612-347 < 347

Why? simplistically, there is more overlap between the orbitals.

Reply 4

Infraspecies

All bonds are the result of atomic orbital overlap.

Sigma bonds have a better atomic orbital overlap than pi bonds, as sigma bonds are the result of a direct overlap, where pi bonds are, instead, the result of a sideways overlap, with the p_z orbitals further away from one another. Hence, they are stronger.
Sigma bonds are also stronger because they have no nodes, whereas pi bonds have a node in the plane normal to the p_z orbitals.

You will never have a separate sigma and pi bond (in the broad majority of cases, with the exception in certain metal complexes, but that's not really useful for you), so it often isn't useful to consider them separately.

It's better to think about how a C=C bond isn't two times as strong as a C-C bond, exactly, so that implies that a pi bond is not as strong as a sigma bond of itself.

Thanks

Pi bonds are weaker because the electron density is above and below the two nuclei that are bonded. Hence the nuclei which are positively charged are exposed to each other, or more so than sigma bonding. Hence there is more repulsion from the two nuclei than there would be if electron density resided between the two nuclei, hence the Pi bond is higher in energy than the sigma bond, requiring less energy to be broken.

Reply 7

username913907

Original post by Infraspecies

You will never have a separate sigma and pi bond (in the broad majority of cases, with the exception in certain metal complexes, but that's not really useful for you), so it often isn't useful to consider them separately.

What transition complexes are you thinking of? I can't think of any purely pi bonded coordination species.

Reply 8

Infraspecies

Original post by JMaydom

What transition complexes are you thinking of? I can't think of any purely pi bonded coordination species.

Carbonyl ligands, π* back bonding with dxz, dxy and dzy orbitals

Reply 9

username913907

Original post by Infraspecies

Carbonyl ligands, π* back bonding with dxz, dxy and dzy orbitals

yeah.... but what about the sigma bonding in that!!!! It's not pure pi bonding!!!

Guessed this might be what you were on about as this popped into my mind when thinking for examples... It's got sigma bonding though!