Question

Using CO as an example, explain the differences in MO theory bonding and VSEPR theory bonding.

Answer 1

Both the MO and VB theories are used to help determine the structure of a molecule. Unlike the VB theory, which is largely based off of valence electrons, the MO theory describes structure more in depth by taking into consideration, for example, the overlap and energies of the bonding and antibonding electrons residing in a particular molecular orbital. While MO theory is more involved and difficult, it results in a more complete picture of the structure of a chosen molecule.

VSEPR is a subset of valence bond. It's just a way of figuring out molecular geometries.The fundamental difference is that valence bond theory describes bonds as local structures, while molecular orbital treats them as more diffuse and spread out over the whole molecule. In VB theory a bond is described by the two adjacent orbitals that go into making the bond. These orbitals are then populated relative to the atoms involved in the relevant bond(s). In MO theory all of the contrribtuing orbitals are considered at once and combined, giving orbitals for the whole molecule, rather than just between two atoms. It basically adds up all of the obitals of the atoms in the molecule to make a new set. These orbitals are then poluated to give the over all electronic structure in the molecule. In VB theory you could describe methane as consisting of 4 separate C-H sigma bonds. In MO theory you would have 4 orbitals that contribute to those bonds, but they would be spread out over the whole molecule and not pertain to any specific C-H bond.