Delocalized Molecular Orbital Theory for 4-Electron 3-Centre Bonding Units

2-3 Delocalized Molecular Orbital Theory for 4-Electron 3-Centre Bonding Units 

In Section 2-2, we have indicated that the standard Lewis representation for a 4-electron 3-centre bonding unit involves resonance between valence-bond structures of the general types (1) and (2), i.e. 

A delocalized molecular orbital description of 4-electron 3-centre bonding is also widely used , and we shall describe three examples of the molecular orbital theory here. 

Because y and b are symmetrically-equivalent atomic orbitals, the delocalized molecular orbitals have the forms given by Eqn. (1), 

The lowest-energy molecular orbital configuration for the 4-electron 3-centre bonding unit is therefore, with the antibonding vgj molecular orbital 

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