VSEPR structures predicted/rationalized

The use of Gillespie s VSEPR theory has allowed the rationalization of these as well as several other structures of noble gas compounds (Fig. 17.3). One of the signal successes of this approach was the early prediction that XeF6 was nonoctahedral (see Chapter 6). The most serious objection lo it is the required promotion of electrons. This has been estimated lobe about 1000 kJ mol 1 (10 eV) or more for xenon, a large amount of energy. Furthermore. J oibit.ils tend lo be diffuse and their importance in nonmetal chemistry is a matter of some controversy (sec Chapter 18). 

The VSEPR model works at its best in rationalizing ground state stereochemistry but does not attempt to indicate a more precise electron distribution. The molecular orbital theory based on 3s and 3p orbitals only is also compatible with a relative weakening of the axial bonds. Use of a simple Hiickel MO model, which considers only CT orbitals in the valence shell and totally neglects explicit electron repulsions can be invoked to interpret the same experimental results. It was demonstrated that the electron-rich three-center bonding model could explain the trends observed in five-coordinate speciesVarious MO models of electronic structure have been proposed to predict the shapes and other properties of non-transition element... 

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