What orbitals form sigma bonds?

1 Answer
Feb 27, 2016

sigma bonds must be made by orbitals that overlap head-on.


POSSIBLE ORBITAL COMBINATIONS TO GENERATE SIGMA MOLECULAR ORBITALS

For simplicity, if we examine only the s, p, and d orbitals, let's suppose that all orbitals we are examining are similar enough in energy to interact.

Let's also suppose that we are ignoring d-d interactions, since we know those should work (they are the same l so it's not so interesting).

Then, orbitals that can capably overlap with each other to form sigma bonds include the following linear combinations:

  • s + p_z -> sigma (bonding)
  • s - p_z -> sigma^"*" (antibonding)
  • s + d_(z^2) -> sigma (bonding)
  • s - d_(z^2) -> sigma^"*" (antibonding)
  • s + d_(x^2-y^2) -> sigma (bonding)
  • s - d_(x^2-y^2) -> sigma^"*" (antibonding)
  • p_z + d_(z^2) -> sigma (bonding)
  • p_z - d_(z^2) -> sigma^"*" (antibonding)
  • p_x + d_(x^2 - y^2) -> sigma (bonding)
  • p_x - d_(x^2 - y^2) -> sigma^"*" (antibonding)
  • p_y + d_(x^2 - y^2) -> sigma (bonding)
  • p_y - d_(x^2 - y^2) -> sigma^"*" (antibonding)

where the z axis is the internuclear axis (i.e. the axis along which the single bond---which is also a sigma bond---is made), and the x and y axes are where you should expect them to be for the Cartesian coordinate system.

We would also suppose that the antibonding molecular orbitals are unoccupied so that the bond is a standard single bond.

HOW TO DEPICT/IMAGINE THESE ORBITAL OVERLAPS

When you sketch these orbital overlaps:

  • All s orbitals are spheres. The only way these can change sign is if the whole thing changes sign.
  • The p_z orbitals can be approximated as dumbbells, regardless of their n, without losing the essence of the sigma MOs generated (head-on overlap). One lobe is the opposite sign to the other.
  • The d_(z^2) look almost like p_z orbitals, except there is a donut in the middle. You can also approximate these as dumbbells, regardless of their n, without losing the essence of the sigma MOs generated (head-on overlap). Both lobes are the same sign.
  • The d_(x^2-y^2) can be approximated as four-leaf clovers, essentially, on the xy-plane, with the lobes aligned along the x and y axes. The opposite lobes along each axis are the same sign. Therefore, they overlap with the p_x and p_y, which also lie long those axes.

Since they are all aligned along the same axis (p_z with d_(z^2), p_x with d_(x^2-y^2), and p_y with d_(x^2-y^2)) AND they are compatible (s with p_z, s with d_(z^2), and s with d_(x^2-y^2)), they form sigma bonding and sigma^"*" antibonding MOs. Since we supposed that only the sigma bonding MO is occupied, we have a single bond.

(Since s orbitals are spheres, it doesn't matter along which axis they bond.)

Of course, there exist f orbitals of some sort that can overlap with s, p_z, d_(z^2), and d_(x^2-y^2) in a sigma fashion, but that's left up to the really motivated chemist to figure out.