Nonoctahedral structure

The first molecular, 6-coordinate complexes which proved to adopt a nonoctahedral structure contain the bidentate, sulfur-donor ligands of the general structures, I and II ... 

Vandium(III) complexes have been the subject of several recent studies 118 -120,341) jjie electronic spectra of these complexes in methylene chloride exhibit the two expected T2g F) — T giF) and TigiP) — Tig F) transitions as well as the two-electron A2g(F) <— Tig(F) transition which is forbidden in octahedral symmetry and generally not observed in solution spectra. The presence of this latter band is indicative of the presence of a nonoctahedral crystal field which is expected for these complexes since the crystal structure... 

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). 

Tn a brilliant paper published in 1893 (38), Alfred Werner established octahedral stereochemistry for the 6-coordinate transition metal ion. In his deliberations Werner also considered both the hexagonal-planar and trigonal-prismatic structures but successfully eliminated these in the cases he investigated. It is a great tribute to Werner that in the 70-odd years since his historic work, countless structural investigations have failed to reveal a truly nonoctahedral, molecular, 6-coordinate complex. It has only been in the past year that an authentic example of nonoctahedral stereochemistry has been found. 

The number of species isoeiectronic with XeF, is quite limited. The anions SbBr, TeCl , and TeBr are octahedral. Both IF and XeF are nonoctahedral." Iodine heptafluoride and rhenium heptafluoride may be considered isoeiectronic with these species if they are all considered to have 14 valence shell electrons of approximately equal steric requirements. Both have a pentagonal bipyramidal structure (see Fig. 6.12). Most interestingly, the XeF anion, formed by the Lewis acid XeF ... 

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