Filling of antibonding orbitals

The case of Ni(CO)4 illustrates that repulsive interactions between doubly occupied orbitals, due to filling of antibonding orbitals are also important. The need to reduce this repulsive interaction often controls the particular bonding configuration of adatoms to surfaces or in cluster compoimds. 

The interaction of a molecule with a metal surface weakens intramolecular bonds and facilitates the dissociation by giving rise to a low activation energy for bond breaking. C-H bond activation in adsorbed hydrocarbons involves the filling of antibonding orbitals by electrons from the metal while the C-H bond stretches. Calculations predicts that the transition state for the formation as well as the dissociation of C-H bonds has rotational freedom and should be characterized as a loose transition state. 

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