Trihalide Anions and some Related Molecules

Chapter 19 Some Electron-Exeess a Bonded Systems 

The delocalized molecular orbital theory for hypervalent molecules involves 3-centre molecular orbitals, and we have given an account of it in Section 2-3(a), for the simplest hypervalent system H . This molecular orbital theory was first proposed in 1951 by Pimentel, and Hach and Rimdle, and has been rrsed subsequently by many workers to describe 4-electron 3-centre o-bonding. Of course, it is also appropriate for ji bonding, as we have indicated in earher chapters. 

For linear, symmetrical I3 the equivalent standard Lewis structures are structures (1) and (2), 

To form the I-I and Xe-F bonds in these structures, the simplest descriptions use the overlap of the 5pa orbitals on the iodine atoms, and the overlap of the xenon 5po orbital with the 2pa orbital of the fluorine. This type of o-orbital overlap for 3-centre bonding is displayed in Fig. 2-4. 

By delocalizing an electron from each of I and F either into vacant bonding I-I and Xe-F orbitals, or into vacant antibonding I-I and Xe-F orbitals, we may obtain the increased-valence structures (5) and (6). 

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