[Dawek]

Hi,

I was studying Molecular Orbital Theory by a pretty qualitative way. Then, i saw s-p crossover, hybridization, etc. But i wonder: what is the minimum energy required for whe consider a interaction between orbitals? For example, the S₂ molecular orbital diagram. I found this: https://i.stack.imgur.com/OoPB7.jpg
But, how could i know if there is no interaction with de sigma antibonding of the 3s AO, and the sigma bonding of the 3p AO? There is a exact value like "if the energy difference is smaller than 13 eV, they will interact"? (13 eV is only a random value).

By the way, can i consider, roughly, that only ionic bounds in ionic compounds like LiF, NaCl, dont have any orbitals interaction, and at the remainder cases (well-known covalent compounds) will always have orbital interaction?

Thanks.

[Corribus]

If two orbitals have compatible symmetry designation, there is interaction. There is no energy cut-off, rather a sliding spectrum of interaction that scales inversely with the difference in energy between the two orbitals (the closer they are together in energy, the larger the interaction). Whether or not you ignore an interaction depends largely on how quantitative you care to be. Accurate quantum chemical calculations using computers consider a large number of interactions. If you are just trying to draw out a diagram to understand broad molecular structure and properties, fewer interactions suffice.

In the ionic limit you can consider there to be very little orbital interaction - the HOMO is almost exclusively (~100%) atomic orbital character of the electronegative atom and the LUMO is almost exclusively (~100%) atomic orbital character of the electropositive atom. This is just for bookkeeping though. "Ionic" versus "covalent" are for the most part artificial designations.

For that matter, molecular orbitals are also artificial constructs. Atomic orbital interactions (LCAO) are based on mathematical approximations anyway.