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

14-2 Approximate Molecular Orbital Theory for 4-Electron 3-Centre Bonding Units 

An approximate 3-centre molecular orbital theory has been used frequently to describe non-symmetrical 4-electron 3-centre bonding units. It involves the con-straction of an (approximate) 3-centre molecular orbital by linearly combining the Y lone-pair orbital with the vacant antibonding A-B orbital of the Lewis structure (4). This gives a 3-centre molecular orbital formulation for the phenomenon of lone-pair delocalization into an antibonding orbital (cf. Sections 7-2 and 7-4 for 6-electron 4-centre bonding). 

If the A-B bond of the standard Lewis structure (4) is described in terms of double occupation of the bonding molecular orbital, the wave-function for this structure is given by Eqn. (1). 

The vacant anbbonding orbital overlaps with the doubly-occupied lone- 

The lowest-energy molecular orbital configuration is then given by Eqn. (3), and this type of configuration has been used either firequendy or implied in qualitative molecular orbital descriptions of 4-electron 3-centre bonding units. Thus  

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