# Why are pi bonds only formed when sigma bonds are formed?

While studying about bonding there was one statement that "pi bonds can only be formed only with sigma bonds" as we know that in double bond there is 1 sigma bond and 1 pi bond but then one question arises: Why is a pi bond formed only when a sigma bond is formed? Is it possible to form a pi bond without any formation of a sigma bond?

A $\pi$ bond has a plane of symmetry along the bond axis. It cannot be formed by s-orbitals; it needs at least p-orbitals to be created. $90\,\%$ of all bonds described some time or another are somehow involving carbon, nitrogen or oxygen. (In fact, I probably underestimated). But these elements can only use p-orbitals to create $\pi$ bonds. To do that, one needs a p-orbital that is ortohogonal to the bond axis. So you run into the problem that you have an orbital pointing in one direction, but want to bond into another direction — hardly optimal, especially since there likely is already another orbital pointing in the direction you need to give a $\sigma$ bond.
Transition metals can use d-orbitals for $\pi$ bonding. They can actually point towards the atom they want to bond with so there is a greater chance of using them due to higher overlap. However, there will usually also be a different orbital pointing directly in the bonding direction which again will bond earlier and would give a $\sigma$ bond.
$\sigma$-bonding MOs tend to have lower energy than $\pi$-bonding MOs so they will be formed first. One explanation is that the $\sigma$-bonding MOs have a lot of $s$-AO character and $s$-AOs have lower energy than $p$-AOs.
I don't know of any double bond that is purely $\pi$-bonding. Basic MO theory suggest that $\ce{C2}$ should have a double bond made of two $\pi$-bonding MOs because the $\sigma$-anti-bonding MO is lower in the energy than the $\pi$-bonding MOs. However, experiment tells us that $\ce{C2}$ has two unpaired electrons so this simple picture cannot be right. This is explained in greater detail here and here.
• In transition metal complexes with 2 metal centers that are bound to each other you can have the situation that the $\pi$-MOs (and sometimes also the $\delta$-MOs) lie energetically below the bonding $\sigma$-MO (all formed from d-orbitals). – Philipp Jun 7 '15 at 11:03
• I have a problem with your statement "One explanation is that the $\sigma$-bonding MOs have a lot of $s$-AO character and $s$-AOs have lower energy than $p$-AOs." While this is true for some cases it is not the main reason for $\sigma$ orbitals being lower in energy than their $\pi$ counterparts (sometimes, like in $\ce{C2}$, it is even the other way round: sp-mixing raises the energy of the higher lying $\sigma$ orbitals so that they lie above the $\pi$ orbitals). The more important reason is the better overlap in $\sigma$-type interactions compared to $\pi$-type interactions. – Philipp Jun 7 '15 at 11:14