Compounds of krypton and radon

   The only binary compound containing Kr is KrF₂. It is a colourless solid which decomposes >250 K, and is best prepared by UV irradiation of a mixture of Kr and F₂ (4 :1 molar ratio) at 77 K. Krypton difluoride is dimorphic. The low-temperature phase, α-KrF₂, is isomorphous with XeF₂ (Figure 1.1a). The structure of the β-form of KrF₂ is shown in Figure 1.1b. The phase transition from β- to α- KrF₂ occurs below 193 K. Krypton difluoride is much less stable than XeF₂.

Fig. 1.1 Unit cells of (a) XeF₂ and (b) β-KrF₂ showing the arrangements and close proximity of molecular units. Colour code: Xe, yellow; Kr, red; F, green.

It is rapidly hydrolysed by water and dissociates into Kr and F₂ at 298K (∆_fH^o(298K) = ‏60.2 kJ mol^-1). We have already exemplified the use of KrF₂ as a powerful oxidizing agent in the syntheses of [XeF₅][AgF₄] and [Xe₂F₁₁]₂[NiF₆] (Section 1.1). Krypton difluoride reacts with a number of pentafluorides, MF₅ (typically in anhydrous HF or BrF₅ at low temperature), to form [KrF]‏[MF₆]^- (M=As, Sb, Bi, Ta), [KrF]‏[M₂F₁₁]^- (M=Sb, Ta, Nb) and [Kr₂F₃]‏[MF₆]^- (M=As, Sb, Ta). In the solid state, the [KrF]⁺‏ ion in [KrF]⁺‏[MF₆]^- (M=As, Sb, Bi) is strongly associated with the anion (e.g. structure 1.1). The [Kr₂F₃]‏ ion (1.1) is structurally similar to [Xe₂F₃]‏ (17.5). The oxidizing and fluorinating powers of KrF₂ are illustrated by its reaction with metallic gold to give [KrF]⁺‏[AuF₆]^-. Few compounds are known that contain Kr bonded to elements other than F. The reactions between KrF₂, RC≡N (e.g. R = H, CF₃) and AsF₅ in liquid HF or BrF₅ yield [(RCN)KrF]⁺[AsF₆]^- with Kr^_N bond formation, and Kr^_O bond formation has been observed in the reaction of KrF₂ and B)OTeF₅(₃ to give Kr)OTeF₅(₂.    Radon is oxidized by halogen fluorides (e.g. ClF, ClF₃) to the non-volatile RnF₂; the latter is reduced by H₂ at 770 K, and is hydrolysed by water in a analogous manner to XeF₂ .

(1.1)                                                          (1.1)