MO Diagrams for Heteronuclear Diatomic Molecules

[ThomasMagnus]

I'm trying to understand how to draw MO diagram for molecules such as CO, HF, and NO. I am finding so much conflicting information on how to order the σ and pi boxes in the diagram. Now, I know that the more electronegative atom's orbitals are going to be lower in energy than the less electronegative atom.

I have learned so far that there are two ways to set up MO diagrams for homo-nuclear diatomic molecules: one was with the pi 2p bonding MO below the σ2p bonding orbital and the reverse for atoms with an atomic number ≥8. How can this be applied to heteronuclear molecules? I keep finding different sources using both ways:

[PLAIN]http://i60.tinypic.com/20tgz61.png[/PLAIN]

[PLAIN]http://i62.tinypic.com/17bdwx.gif[/PLAIN]

Are both correct then?

Thanks!

 

[DrDu]

The correct order of the orbitals depends on the s-p energy gap in both atoms, the relative energetic distance of the s and p orbitals on both atoms, respectively, the orbital overlapp etc. So to obtain the correct ordering will require a calculation.
You may try this yourself using e.g. some Extended Hueckel Calculators:
http://www.chembio.uoguelph.ca/educmat/chm715/software.htm
or
http://yaehmop.sourceforge.net/

On the other hand, the correct ordering is often not important, e.g. when both orbitals whose relative energy is unclear are both occupied.