Xenon orbital model

Figure 5. Shapes of the XeF ions based on steric activity of the nonbonding xenon valence-electron pairs. (Arrows indicate directions of maximum polarizing effect.) [These models represent the nonbonding xenon electrons in a formalistic way. In the Xe-F case the model cannot be realistic since such a cation has cylindrical symmetry. The postulated axial polarizing behavior can also be seen to be a consequence of Xe-F bond formation. Thus we can synthesize XeF by bringing F ( D) up to the spherical Xe atom. If we use a p-orbital pair of electrons of the Xe atom to form the Xe-F bond, the electron density will be diminished trans to the bond.]...

This procedure provides a model of the xenon atom which accounts only for the manifold of singly excited states based on the lowest ionic core, P3/2- For all rare gases, a second manifold of states converges to the next spin-orbit component of the ion, the Pi/2 state. For example, these two ionization limits in xenon are separated by 1.3 eV corresponding to different total angular momenta, J, of the 5p configuration. The lower ionization potential is 12.15 eV. We assume that multiphoton excitations into these two manifolds are very weakly coupled so they can be treated separately. This assumption is reasonable because once one of the electrons is excited outside a particular core configuration, transitions... 

The large positive shift and the parabolic behaviour of the 5 = f(N) curves in the case of divalent cations was attributed first by Fraissard at al. [2] to the high polarisability of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2+ cations. Later, Cheung et al. [5] proposed a model to explain the strong adsorption of xenon in zeolites with 2+ cations (Ca2+, Mg2+, Ba2+). It consists in extending the electron attraction described above to the point where an electron is transferred from the xenon to the cation. This model suggests that a partial bond between the xenon atom and the 2+ cation is formed by donation of a xenon 5p electron to the empty s-orbital of the 2+ cation. 

This would mean that there are 10 electrons in the valence shell of the Ng atom in xenon difluoride or krypton difluoride and 12 or 14 electrons for xenon tetrafluoride or hexafluoride, and even more for the octafluoroxenate ion, [XeFs] . Since one s and three p orbitals can accommodate only eight electrons, this would require the participation of d orbitals. In fact, the currently favored model uses only s and p atomic orbitals [16]. For example, XeF2 can be constmcted with a three-center-two-electron (3c-2e) bond, like NF5 (Chapter 4, Fig. 4.5) without using d orbitals (Fig. 5.1). Perhaps one should not worry much about which orbitals are involved, because as has been pointed out, bonding is not an observable quantity only bonding distancies and electron density are amenable to observation although... 

Molecules of noble gas compounds have shapes in excellent agreement with VSEPR theory, although to form two-electron bonds, xenon must promote valence electrons to higher-energy orbitals such as 5d. This model can be avoided by a molecular orbital treatment which, in Xep2, forms three-centre bonds from xenon 5p and fluorine 2p orbitals. The Xe—F bonds then have an order much less than one. The experimentally determined bond lengths are in better agreement with the two-electron, two-centre bonds of VSEPR theory. 

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