This week Elvis Shoko, Seth Olsen, and I read more of the beautiful review article, by Roald Hoffmann, A chemical and theoretical way to look at bonding on surfaces.
One of the key ideas I learnt was how chemical bonding between a metal surface and a molecule can have a completely different physical mechanism than between just molecules.
The left side shows how the interaction between two molecules due to the two filled highest orbitals is repulsive.
In contrast, similar interactions between a molecular orbital and a filled metal orbital can be attractive because once the metal-molecule interaction is strong enough to push the anti-bonding orbital above the Fermi energy, there can be charge transfer to unoccupied metal states.
The Figure above shows the energy of interaction as a function of the distance of the molecule from the surface. This also provides an understanding of the energy barrier to chemisorption of the molecule on the surface.
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