Hi all

I am having trouble finding the MO order for heteronuclear diatomic molecules:

In my textbook, heteronuclear order is listed as:
1o 2o 3o 4o 1pi 5o 2pi 6o

where o = sigma

When I first saw it, I was confused; how can the above be correct? If one were to calculate bond order, wouldn't it just keep increasing?

Now, for NO and CO, I BELIEVE MO order is:
1o 2o* 3o 4o* 1pi 5o 2pi*

But why is it like this? Is there a general order like in homonuclear diatomic molecules?
Is my textbook incorrect?

Regards

Hello!

I understand that you are having trouble understanding the molecular orbital (MO) order for heteronuclear diatomic molecules. Let me explain how the MO order is determined and why it may differ for different molecules.

In heteronuclear diatomic molecules, the MO order is determined by the atomic energies of the constituent atoms and their overlapping. The order depends on the electronegativity difference between the two atoms and the relative energy levels of their atomic orbitals.

The MO order you provided (1o 2o 3o 4o 1π 5o 2π 6o) is a general trend for heteronuclear diatomic molecules. The "o" represents sigma (σ) orbitals, and the "π" represents pi (π) orbitals.

Now, let's address your confusion about the increasing bond order. Bond order is a measure of the strength and stability of a chemical bond. It is calculated by taking the difference between the number of bonding electrons and the number of antibonding electrons, divided by two.

In the general MO order for heteronuclear diatomic molecules, the bond order does not necessarily increase linearly because the energies of MOs can be different. Sometimes, the energy of a higher MO can be lower than the energy of a lower MO. Therefore, the bond order may decrease for certain MOs, leading to a non-increasing trend.

For NO and CO, the MO order you mentioned (1o 2o* 3o 4o* 1π 5o 2π*) is indeed correct. Here, the asterisk (*) represents the antibonding states. The asterisked MOs have different energies compared to their non-asterisked counterparts, resulting in a non-linear MO order.

It's important to note that the MO order can vary for different molecules due to differences in electronegativity, atomic energies, and atomic orbital overlaps. So, it's possible that your textbook may have presented a general trend, but it may not apply to all heteronuclear diatomic molecules.

To determine the MO order for a specific heteronuclear diatomic molecule, you would need to consider the specific atomic energies and electronegativities of the atoms involved. You can refer to specialized textbooks, databases, or computational methods to obtain detailed molecular orbital diagrams for specific molecules.

I hope this explanation helps clarify your doubts! If you have any further questions, feel free to ask.