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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: blaisem on January 26, 2014, 11:41:28 AM

Title: Ordering of the Molecular Orbital energies in an octahedral complex
Post by: blaisem on January 26, 2014, 11:41:28 AM

Hello, I was wondering why the t₁u* orbital is higher energy than the a₁g* orbital, while the opposite is true for their corresponding bonding molecular orbitals.  For the bonding case, I understand that the overlap is better, leading to a lower energy a₁g orbital, but the reverse is true for the antibonding case?

Thank you!

Attachment from Wikipedia commons

Title: Re: Ordering of the Molecular Orbital energies in an octahedral complex
Post by: blaisem on February 01, 2014, 01:48:31 PM

This is a response to my own question, for which I have a theory that I think might be right.  The energies of the initial orbitals in qualitative molecular orbital diagram are ordered based on their electronegativity or closeness to the nucleus.  If the p-orbital is higher in initial energy, and its splitting is similar in magnitude to the s-orbital, then it's possible that the t_1u bonding orbital is higher in energy than the a_1g bonding orbital, while the opposite is true for their antibonding orbitals.

Seems to make a lot of sense.  I just didn't want to leave the question unanswered if I thought I had a reasonable explanation.  It's all pretty qualitative, so maybe I am just thinking things through too much for this case.

Title: Re: Ordering of the Molecular Orbital energies in an octahedral complex
Post by: Irlanur on February 21, 2014, 08:28:32 AM

I assume that the diagram is not quantitavley correct, that's why I wouldn't hurt my head with too lot of thinking... It's also not important at all for the chemistry. An Answer would need more calculations.