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Why do homonuclear and heteronuclear MO diagrams use different orbital labelling systems?

Question asked by Allan Carmen on Aug 6, 2018

In a textbook I am using, the homonuclear Molecular Orbital Theory Diagrams label anti-bonding orbitals with a (*), and label both corresponding bonding and anti-bonding sigma-orbitals with the same number (e.g. 1σg then 1σu*).

 

The heteronuclear MO Theory Diagrams, however, don't always label the anti-bonding orbitals with a (*) and label the bonding and anti-bonding sigma-orbitals with sequential numbers. (e.g. 3σ then 4σ*; or 3σ, 4σ, 5σ then 6σ).

 

I've attached pictures to make it clearer.

 

I understand why the parity labels aren't used, but I don't understand why the numbering system changes or why the * isn't always included.

 

If anyone could explain this I would be really appreciative, thank you!

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