Hypervalent compounds of the noble gases

The molecular structures of the five homoleptic noble gas compounds that are stable enough to be studied in the gas phase are shown in Fig. 19.1. If the noble gas atoms are assumed to form single electron pair bonds to the fluorine atoms, and if the Xe-O bonds in Xe04 are assumed to be double (four electron bonds), then the K and Xe atoms in the trifluorides are surroimded by 10 electrons in the valence shell, the Xe atom in XeP4 by 12, the Xe atom in XeP6 by 14, and in Xe04 by 16 electrons. AU these compounds may thus be described as containing hypervalent noble gas atoms. 

Problem 19.1 Discuss the structure and bonding in Krp2, Xep2 and Xep4 in terms of the VSEPR and MO models. 

The mean Xe-F bond distances decrease linearly with the number of fluorine atoms from 198 in the difluoride to 189 pm in the hexafluoride. This decrease of the bond distances 

Alternatively the observed gas electron diffraction patterns may be interpreted in terms of an extremely flexible octahedral equilibrium structure in which the energy required to deform the molecule under Csv symmetry is so low compared to the thermal energy that a significant fraction of the molecules have instantaneous structures far from octahedral symmetry during the brief ( 10 s) electron scattering process. 

Very recent, state of the art quantum chemical calculations led to the conclusion that the energy difference between octahedral and capped octahedral stmctures is less than 1 kJmol [6]. This difference is too small to allow us to draw a conclusion as to which structure is the lower. The calculations and experimental evidence thus agree that the energy difference between octahedral and capped octahedral structures is so small that XeFe gas at a given instant will contain a significant number of molecules with either geometry, but we still do not know which of them represents the equilibrium structure. 

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