Q5(a). Octafluoroxenate(VI): preparation, structure and bonding

1. Preparation: XeF₆ is a fluoride-ion acceptor (Lewis acid). With a fluoride source it gives XeF₇⁻ or XeF₈²-, depending on the proportion of F⁻: 3pt] XeF₆ + CsF → Cs[XeF₇] 2Cs→Cs₂⁺ XeF₆ 3pt] Nitrosyl fluoride gives the nitrosonium salt XeF₆ + 2NOF → (NO)₂[XeF₈]. Large cations (Cs⁺, Rb⁺, NO⁺) stabilise the large anion through lattice energy; the solids are stable well above room temperature but are hydrolysed by moisture.
2. Electron count: Xe 8 + 8 F (one each) + 2 (charge) = 18 electrons = 9 pairs: 8 bonding pairs and 1 lone pair (Xe oxidation state +6).
3. Observed structure: an approximate square antiprism (two parallel squares of F, one turned 45° to the other, D_(4d)). One square face is slightly larger, so the true symmetry is nearer C₄ᵥ.
4. The lone pair: VSEPR predicts a definite site for the ninth pair, but the antiprism has none. The lone pair is stereochemically inactive: it stays in the spherical Xe 5s orbital, while eight F crowd round Xe. Its small effect may be the slight enlargement of one face.
5. Bonding: the Xe–F bonds are electron-rich, multicentre bonds (three-centre four-electron type using Xe 5p and F 2p orbitals) with large ionic character, Xe^(δ+)–F^(δ-); no 5d participation is needed. High coordination is possible because F is small and very electronegative.

Application. xenon fluorides and their anions are clean fluorinating agents (by-product only Xe); XeF₆ is separated from XeF₄ and XeF₂ by forming involatile NaF.XeF₆-type adducts and then heating them.