Antibonding orbital fragments

The theoretical chemical application of surface chemical bonding theory, highlighted next, is related to formal chemisorption theory as developed in surface physics, but concentrates on quantum chemical concepts as the electron distribution over bonding and antibonding orbital fragments [5, 6]. It will be seen that both approaches complement each other. The notion of a surface molecule relates to the surface physicists concept of surface state. 

When the d valence electron band is nearly completely filled, interaction with the doubly occupied CO 5o orbital, leading to a significant fraction of occupied antibonding orbital fragments a between adsorbate and surface atoms, will be repulsive. This Pauli repulsion is proportional to the number of surface atom neighbours and hence is a minimum in atop coordination. This counteracts the... 

The covalent interaction with the adatom 2p orbitals increases with decreasing surface d valence electron occupation, because fewer antibonding orbital fragments then become occupied. The adsorption energy of atoms varies much more strongly with d valence electron occupation than that of molecules because compensating effects occur in the surface chemical bond of molecules. As we discussed above for CO, variation in the interaction with the 5a orbital is partially compensated for by changes in the interaction with the 2jc orbital. 

In the adsorption of ammonia on the surface, the NH3 2p lone-pair molecular orbital interacts with a transition-metal smface atom to form bonding and antibonding orbital fragments. The resulting 4d 2 electron distributions are also shown in Fig. 3.3b. 

If the d-valence electron band and the 5cr-orbital are both completely filled with electrons, the interaction energy will be repulsive since Pauli repulsion is proportional to the overlap of S and /3. When the d-electron valence bond is partially empty, this repulsive interaction is decreased because the antibonding orbital fragments become less occupied. The decrease in repulsive energy with a decrease in number of metal-atom neighbors of the surface atom involved in the adsorbate bond relates to an increase in the number of empty antibonding orbitals, determined by the electron density between ( ax (see Fig. 3.10). 

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