CO MOs

I talked about the nitrogen MO scheme a while ago. Let's see what happens if you take a proton from one of the nuclei and give it to the other one.

The molecular orbitals were calculated from a minimal basis with AM1 parametrisation in Arguslab. They are pretty close to the ones of nitrogen. The difference is that lower energy MOs are rather O-centered and higher energy MOs rather C-centered. This makes sense because of the fact that the atomic orbitals of oxygen are lower in energy. It can be seen that the HOMO and the LUMO [1] are centered on the carbon. This causes the strong σ-donor and π-acceptor characteristics of CO.

---

The table has the LCAO coefficients for the different orbitals. The color coding shows if orbitals have a bonding or antibonding interaction [2]. It can be seen the the lowest energy AO (O-2s) has the strongest impact on the lowest energy MO, and that the highest energy AO (C-2p) has the strongest impact on the highest energy MO.

C-2s	C-2pσ	C-2pπ	O-2s	O-2pσ	O-2pπ
0.29	0.721		-0.234	0.585
		-0.865			0.502
		0.865			-0.502
0.583	-0.619		0.105	0.515
		-0.502			-0.865
		0.502			0.865
-0.634	-0.036		0.523	0.568
-0.416	-0.31		-0.813	0.264

CO is usually drawn with a formal positive charge on the oxygen and a negative charge on the carbon.
Formal charge and polar bonds work in opposite ways. This results in a very small dipole moment of .1 Debye. Its direction was argued over for a while. Now theoristical chemists are pretty sure that at equilibrium distance the carbon atom is the negative end.

---

[1] Theorists always seem to be kind of careful when talking about unoccupied MOs. In order to sound serious I have to do that, too: One has to remember the fact that unoccupied MOs are even less real than occupied ones. For some reason they are pretty good if you use a minimal base. But as soon as you move from plain LCAO to using more basis functions you will just get a lot of "virtual" MOs close together in energy with no physical significance.

[2] For σ-MOs the color corresponds to the sign that the wave function has in between the nuclei, for π-MOs it corresponds to the sign in the positive x-direction of the atom.