The Antibonding Effect

It is shown that the consequence of filling both a bonding molecular orbital (MO) and its antibonding counterpart leads to a total orbital energy greater than that of the separated atoms. The resulting antibonding effect can be buffered if the antibonding MO mixes with higher empty MOs of the same symmetry. These considerations explain why Be 2 has a weak covalent bond, much stronger than in He 2 . The antibonding effect also helps to explain the weakness of the F-F, O-O, and N-N single bonds. It is also useful in dealing with the stereochemistry of d n transition metal ions ( n > 7); the favored coordination geometries are those that minimize the antibonding effect, or which allow its effective buffering.