In (I think all) of the molecular orbital (MO) diagrams I have seen, I have observed that the number of antibonding molecular orbitals is the same as the number of bonding molecular orbitals.

Here are some for example:

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I know that the number of atomic orbitals equals the number of molecular orbitals, but I have not heard of an explicit rule stating that the number of bonding molecular orbitals is equal to the number of antibonding molecular orbitals. Is this always true? If not, can someone please give a counter example?

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    $\begingroup$ Cyclopentadienyl is a counter example, as are other odd-numbered conjugated ring anions. $\endgroup$ – Tyberius Feb 11 at 18:32
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    $\begingroup$ Have you looked at the pi orbitals of aromatic ions? $\endgroup$ – Oscar Lanzi Feb 11 at 19:05
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    $\begingroup$ Yes, they just escaped from my memory when I was thinking about this question. $\endgroup$ – DrPepper Feb 11 at 19:06

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