Crystal Structures of Xenon(VI) Salts: XeF5Ni(AsF6)3, XeF5AF6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au), and XeF5A2F11 (A = Nb, Ta)

Experiments on the preparation of the new mixed cations XeF5M(AF6)3 (M = Cu, Ni; A = Cr, Nb, Ta, Ru, Rh, Re, Os, Ir, Pt, Au, As), XeF5M(SbF6)3 (M = Sn, Pb), and XeF5M(BF4)x(SbF6)3-x (x = 1, 2, 3; M = Co, Mn, Ni, Zn) salts were successful only in the preparation of XeF5Ni(AsF6)3. In other cases, mixtures of different products, mostly XeF5AF6 and XeF5A2F11 salts, were obtained. The crystal structures of XeF5Ni(AsF6)3, XeF5TaF6, XeF5RhF6, XeF5IrF6, XeF5Nb2F11, XeF5Ta2F11, and [Ni(XeF2)2](IrF6)2 were determined for the first time on single crystals at 150 K by X-ray diffraction. The crystal structures of XeF5NbF6, XeF5PtF6, XeF5RuF6, XeF5AuF6, and (Xe2F11)2(NiF6) were redetermined by the same method at 150 K. The crystal structure of XeF5RhF6 represents a new structural type in the family of XeF5AF6 salts, which crystallize in four different structural types. The XeF5A2F11 salts (M = Nb, Ta) are not isotypic and both represent a new structure type. They consist of [XeF5]+ cations and dimeric [A2F11]− anions. The crystal structure of [Ni(XeF2)2](IrF6)2 is a first example of a coordination compound in which XeF2 is coordinated to the Ni2+ cation.


Introduction
The synthesis of XeF 6 was first described in 1962 [1]. It was prepared by the reaction between xenon and fluorine (molar ratio 1:20) at 700 • C and a pressure of~200 bar F 2 . Later systematic studies showed that XeF 6 can be prepared under milder conditions (molar ratio 1:10; 200 • C; and a total pressure of 33 bar) [2]. In general, XeF 6 is prepared by heating a mixture of Xe and F 2 (molar ratio 1:20) at 300 • C and a total pressure of~50 bar [3].
In the presence of a NiF 2 catalyst, XeF 6 forms explosively from the gaseous mixture of xenon and fluorine in a molar ratio of 1:5 already at 120 • C [4]. An alternative method with high yield for the preparation of high-purity XeF 6 is the reaction between Xe and F 2 at low pressure and high filament temperature in a "hot wire" reactor [5]. At room temperature, XeF 6 is solid (T m.p. = 49.48 • C, T b.p. = 75.57 • C) with a vapor pressure of about 0.03 bar at 23 • C [6]. The color of solid XeF 6 has been reported to range from colorless to intense yellow. An explanation for these color variations of solid XeF 6 is not apparent to date [7]. Liquid XeF 6 and its vapors are yellow-green [6]. There are six, possibly seven, different modifications of solid XeF 6 [7]. In CF 2 Cl 2 /SO 2 ClF solution, XeF 6 exists as a tetramer (XeF 6 ) 4 [8], while in the gas phase XeF 6 exists as a monomer [9][10][11]. The presence of a sterically active free valence electron pair on Xe leads to the XeF 6 molecule being fluxional [12]. Consequently, the structure of monomeric XeF 6 has been a major challenge for theoretical computational chemistry [12][13][14][15][16][17]. Experiments have clearly shown that XeF 6 is not octahedral (O h ) but most likely has the shape of a slightly distorted octahedron (C 3v ), naturally in a dynamical form [18]. Both conformers are energetically very close to each other [15]. Because of the low barrier of interconversion, XeF 6 is a highly fluoxinal molecule that rapidly converts between the 8-fold degenerate C 3v structures via the octahedral minimum even at low temperatures [15]. In further studies, XeF 6 was theoretically shown to exhibit a genuine quantum mechanical fluorine tunneling rearrangement, where it "jumps" The proposed synthetic methods lead to a mixture of substances. The work carried out is an X-ray diffraction study of some crystal phases of these mixtures. In addition, some of the products were also confirmed by Raman spectroscopy (Supplementary Materials).
Reactions between XeF 2 , MF 2 (M = Cu, Ni), AsF 5 , and UV-irradiated F 2 in anhydrous hydrogen fluoride (aHF) resulted in clear colorless (Cu) and yellow (Ni) solutions (Table S1). In the case of nickel, single crystals of XeF 5 Ni(AsF 6 ) 3 were obtained upon crystallization (Table 1), while in the case of copper a mixture of single crystals of XeF 5 AsF 6 [41] and CuFAsF 6 [42] was observed in the crystallization product. In all other experiments where mixtures of XeF 2 /MF 2 (M = Ni, Cu) with addition of AF 3 (A = Cr, Au), AF 5 (A = Nb, Ta), or metal powder A (A = Re, Ru, Rh, Os, Ir, Pt) were treated with UV-irradiated F 2 (Table S1), the insoluble material did not disappear even after several days. For crystallization, the clear supernatant, which contained no visible sediments, was decanted into the side arm of the double-arm crystallization vessel. Only single crystals of XeF 5 AF 6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au) and XeF 5 A 2 F 11 salts (A = Nb, Ta) were grown from the corresponding solutions (Table S1). In the case of Ru and Pt, traces of O 2 AF 6 salts (A = Ru, Pt) [43,44] were also present. Although the remaining insoluble solids were not characterized, it can be assumed that they probably consisted of M(AF 6 ) 2 salts (M = Ni, Cu; A = Nb, Ta, Ru, Rh, Ir, Pt, Au). Of these, only Cu(AuF 6 ) 2 and Ni(AuF 6 ) 2 [45] are known, while the others have not yet been synthesized. Similar to the M[AuF 6 ] 2 salts, unlike the M(AF 6 ) 2 salts (A = As, Sb) [46], they are probably not well soluble or they are insoluble in anhydrous HF. For the M(AF 6 ) 2 salts (M = Ni, Cu; A = Nb, Ta, Ru, Rh, Os, Ir, Pt), their lattice energy appears to overcome the solvation energy. This would explain their insolubility in aHF and the preferential formation of mixtures of M(AF 6 ) 2 (insoluble in aHF) and XeF 5 AF 6 (soluble in aHF) instead of XeF 5 M(AF 6 ) 3 salts (M = Cu, Ni; A = Nb, Ta, Ru, Rh, Ir, Pt, Au). Table 1. Summary of crystal data and refinement results for XeF 5 AF 6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au).
In the XeF 2 /NiF 2 /Re/UV-irradiated F 2 system, few single crystals of (Xe 2 F 11 ) 2 (NiF 6 ) were detected as crystallization product (Table S1). When pure Re powder is treated with UV-irradiated F 2 in aHF, it oxidizes to volatile ReF 6 [47]. The latter does not react in aHF even with a very good fluorine ion donor such as CsF [47]. In the XeF 2 /NiF 2 /Re/UVirradiated F 2 /aHF system, all the Re is oxidized to inert gaseous ReF 6 . When the NiF 2 /F 2 reaction mixture in aHF is irradiated with UV light, the particles of pale yellow-green NiF 2 , which is insoluble in aHF, turn black. This indicates that NiF 2 is first fluorinated to NiF 2+x (x ≤ 1) [47]. When only XeF 2 (Xe II ) is present without other compounds, XeF 2 is oxidized to Xe IV (XeF 4 ) by elemental fluorine under UV light in aHF [47]. Our experiment has shown that in the presence of Ni(II) and Xe(II) as reagents, oxidation to Ni(IV) and Xe(VI) occurs, giving (Xe 2 F 11 ) 2 (NiF 6 ) [40]. The A' 2 NiF 6 salts (A' = Li, Na, K, Cs) can also be prepared by oxidation of NiF 2 at about 20 • C by sunlight or UV-irradiated F 2 in liquid aHF containing dissolved alkali metal fluorides (LiF, NaF, KF, CsF) [48].
In the XeF 2 /CuF 2 /Os/UV-irradiated F 2 /HF system, a colorless solution was observed over white insoluble material. Upon cooling the reaction vessel to 77 K, a strong yellow coloration of the solid aHF was observed, indicating the presence of yellow OsF 6 [49]. When crystallized from the decanted clear solution, only two small colorless crystals were formed. One of them was detected as XeF 4 by Raman spectroscopy, while the other exploded on the diffractometer goniometer.
The crystal structures of XeF5AF5 (A = Nb, Ru, Pt, Au) determined at 150 K are the same as at room temperature. The crystal structure of XeF5IrF5 agrees with the XPD data [39], while the crystal structures of XeF5TaF6 and XeF5RhF6 have been determined for the first time. Based on the results of this study and the data known from the literature, the crystal structures of the XeF5AF6 salts (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) can be classified into four types of structures (type I, II, III, and IV).

Type I; XeF5AF6 (A = Nb, Ta, Ru, Os, Ir, Pt, Sb) Salts
The crystal structure of XeF5PtF6 was described as the first example of type I [24]. The crystal structures of the salt XeF5AF6 (A = Nb, Ta, Ru, Os, Ir, Sb) are isotypic to this type (Tables 1 and 3). The members of type I crystallize in the orthorhombic Pnma space group, in which the asymmetric structural unit consists of a crystallographically unique [XeF5] + cation and [AF6] -anion ( Figure 1).  Each [XeF 5 ] + cation exhibits the typical geometry, i.e., a pseudo-octahedral AX 5 E VSEPR arrangement of the bond pairs (X) and the lone pair (E). The Xe-F ax bonds are shorter than the other four Xe-F eq distances (Table 4, Figure 2). Each XeF 5 unit forms four secondary contacts with the fluorine atoms of four AF 6 groups ( Figure 2). Each [AF 6 ] − anion participates in four secondary contacts with four different XeF 5 groups ( Figure 2).
The crystal structures of XeF5AF5 (A = Nb, Ru, Pt, Au) determined at 150 K are the same as at room temperature. The crystal structure of XeF5IrF5 agrees with the XPD data [39], while the crystal structures of XeF5TaF6 and XeF5RhF6 have been determined for the first time. Based on the results of this study and the data known from the literature, the crystal structures of the XeF5AF6 salts (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) can be classified into four types of structures (type I, II, III, and IV).

Type I; XeF5AF6 (A = Nb, Ta, Ru, Os, Ir, Pt, Sb) Salts
The crystal structure of XeF5PtF6 was described as the first example of type I [24]. The crystal structures of the salt XeF5AF6 (A = Nb, Ta, Ru, Os, Ir, Sb) are isotypic to this type (Tables 1 and 3). The members of type I crystallize in the orthorhombic Pnma space group, in which the asymmetric structural unit consists of a crystallographically unique [XeF5] + cation and [AF6] -anion ( Figure 1).

Type III; XeF 5 AF 5 (A = Rh) Salts
The crystal structure of XeF 5 RhF 6 is the only representative of type III (Tables 1 and 4  The crystal structure of XeF5AsF6 was first determined at room temperature [41] and later redetermined at 150 K [37]. When the crystal first measured at 150 K was cooled to 80 K and data were collected, no phase transition was observed [37]. In both cases, only a monoclinic phase was obtained (150 K; P21/c, Z = 4, a = 5.8222 (6) Å, b = 16.3566 (15)   The geometry of the [XeF 5 ] + in the type III salts is similar to that in the type I and II compounds (d(Xe-F ax ) < d(Xe-F eq ); Table 4, Figure 6), while the nature of the secondary interaction contacts between the [XeF 5 ] + cations and the [AF 6 ] − anions is different. In type III, each [XeF 5 ] + cation forms four secondary contacts with the fluorine atoms of three AF 6 groups, while each [AF 6 ] − anion participates in four secondary contacts with three different XeF 5 groups ( Figure 6).    The crystal structure of XeF 5 AsF 6 was first determined at room temperature [41] and later redetermined at 150 K [37]. When the crystal first measured at 150 K was cooled to 80 K and data were collected, no phase transition was observed [37]. In both cases, only a monoclinic phase was obtained (150 K;   All the compounds listed in Table 8   All the compounds listed in Table 8

General Considerations for XeF5AF6
Salts (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) Table 9 lists the effective ionic radii r(A 5+ ) (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) for coordination number six [56], the formula units (molecular) volumes VFU of LiAF6, CsAF6, and XeF5AF6, and the average A-F bond lengths in LiAF6 and XeF5AF6. The molecular volumes of LiAF6, CsAF6, and XeF5AF6 are shown in Figure 11. The crystal structures of LiAF6 (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, and Au) were determined by synchrotron X-ray powder diffraction at 299 K [57]. The crystal structures of LiAsF6 and LiSbF6 were determined by X-ray diffraction on powdered material and single crystals, respectively, at room temperature (RT) [58,59]. With the exception of CsAsF6 and CsSbF6, whose complete crystal structures were determined on single crystals at RT, only unit cells determined at RT are available for the other CsAF6 salts [60]. The crystal structures of CsAF6 (A = Rh, Pt, Ir, Os, and Au) were also determined on single crystals at 150 K [61]. The crystal structures of XeF5AF6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au, As [37], Sb [25]) were determined at 150 K and some of them also at RT (A = As [41], Au [39], Pt [24], Ru [38], Sb [37], Nb [30]).   Table 9 lists the effective ionic radii r(A 5+ ) (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) for coordination number six [56], the formula units (molecular) volumes V FU of LiAF 6 , CsAF 6 , and XeF 5 AF 6 , and the average A-F bond lengths in LiAF 6 and XeF 5 AF 6 . The molecular volumes of LiAF 6 , CsAF 6 , and XeF 5 AF 6 are shown in Figure 11. The crystal structures of LiAF 6 (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, and Au) were determined by synchrotron X-ray powder diffraction at 299 K [57]. The crystal structures of LiAsF 6 and LiSbF 6 were determined by X-ray diffraction on powdered material and single crystals, respectively, at room temperature (RT) [58,59]. With the exception of CsAsF 6 and CsSbF 6 , whose complete crystal structures were determined on single crystals at RT, only unit cells determined at RT are available for the other CsAF 6 salts [60]. The crystal structures of CsAF 6 (A = Rh, Pt, Ir, Os, and Au) were also determined on single crystals at 150 K [61]. The crystal structures of XeF 5 AF 6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au, As [37], Sb [25]) were determined at 150 K and some of them also at RT (A = As [41], Au [39], Pt [24], Ru [38], Sb [37], Nb [30]). Table 9. Effective ionic radii r(A 5+ ) (A = Nb, Ta, Ru, Rh, Os, Ir, Pt, Au, As, Sb) for coordination number six (Å), formula unit (molecular) volumes V FU (Å 3 ) of LiAF 6 , CsAF 6 , and XeF 5 AF 6 , and average A-F bond lengths d av [A-F] (Å) in LiAF 6 and XeF 5 AF 6  Although the formula unit volumes V FU of the LiAF 6 and CsAF 6 salts ( Figure 11) show a similar trend, this is not the case for the XeF 5 AF 6 salts, with the V FU of XeF 5 RhF 6 and XeF 5 AuF 6 being particularly prominent. For LiAF 6 and CsAF 6 , the V FU are smallest for the As, Rh, and Au salts and largest for the Sb, Nb, and Ta salts. For the [XeF 5 ] + salts, the V FU of XeF 5 AuF 6 is almost identical to the V FU of XeF 5 TaF 6 , while XeF 5 RhF 6 has the smallest value V FU of all XeF 5 AF 6 salts (Table 9, Figure 11).  Type  II  III  II  I  I  I  I  I  I  I   Although the formula unit volumes VFU of the LiAF6 and CsAF6 salts ( Figure 11) show a similar trend, this is not the case for the XeF5AF6 salts, with the VFU of XeF5RhF6 and XeF5AuF6 being particularly prominent. For LiAF6 and CsAF6, the VFU are smallest for the As, Rh, and Au salts and largest for the Sb, Nb, and Ta salts. For the [XeF5] + salts, the VFU of XeF5AuF6 is almost identical to the VFU of XeF5TaF6, while XeF5RhF6 has the smallest value VFU of all XeF5AF6 salts (Table 9, Figure 11).

Crystal Structures of XeF5A2F11 (A = Nb, Ta, Sb)
In the XeF6-AF5 (A = Nb, Ta) system, only the salts XeF5AF6 and Xe2F11AF6 have been known so far [30,[62,63]. The salts XeF5Nb2F11 and XeF5Ta 2F11 were prepared for the first time in this study. As in the case of XeF5Sb2F11 [25] (Figure 12) the crystal structures of XeF5A2F11 (A = Nb, Ta) consist of discrete [XeF5] + cations and dimeric [A2F11] − anions interacting through secondary fluorine bridge XeF-A contacts (Figures 13 and 14). Each  Figure 11. Formula unit volumes of LiAF 6 (for A = Rh, Au, Pt, Ir, Ru, Os, Nb, Ta at 299 K and for A = As, Sb at room temperature), CsAF 6 (all data at room temperature), and XeF 5 AF 6 salts (all data at 150 K).

Crystal Structures of XeF 5 A 2 F 11 (A = Nb, Ta, Sb)
In the XeF 6 -AF 5 (A = Nb, Ta) system, only the salts XeF 5 AF 6 and Xe 2 F 11 AF 6 have been known so far [30,62,63]. The salts XeF 5 Nb 2 F 11 and XeF 5 Ta 2 F 11 were prepared for the first time in this study. As in the case of XeF 5 Sb 2 F 11 [25] (Figure 12) the crystal structures of XeF 5 A 2 F 11 (A = Nb, Ta) consist of discrete [XeF 5 ] + cations and dimeric [A 2 F 11 ] − anions interacting through secondary fluorine bridge Xe···F-A contacts (Figures 13 and 14). Each crystal structure of the XeF 5 A 2 F 11 salts (A = Sb, Nb, Ta) represents a unique example (Tables 2 and 5).      In XeF5Nb2F11, each [XeF5] + cation forms three secondary contacts with the fluorine atoms of two Nb2F11 groups, whereas in XeF5Sb2F11 and XeF5Ta2F11, each [A2F11] -anion (A = Nb, Ta) participates in four secondary contacts with three different A2F11 dimers ( Figures  12-14). In all three salts, the Xe-Fax bonds are shorter than the other four Xe-Feq distances ( Table 5). The A-Fb-A bridge (Table 5)      In XeF5Nb2F11, each [XeF5] + cation forms three secondary contacts with the fluorine atoms of two Nb2F11 groups, whereas in XeF5Sb2F11 and XeF5Ta2F11, each [A2F11] -anion (A = Nb, Ta) participates in four secondary contacts with three different A2F11 dimers ( Figures  12-14). In all three salts, the Xe-Fax bonds are shorter than the other four Xe-Feq distances ( Table 5). The A-Fb-A bridge (Table 5)   In XeF 5 Nb 2 F 11 , each [XeF 5 ] + cation forms three secondary contacts with the fluorine atoms of two Nb 2 F 11 groups, whereas in XeF 5 Sb 2 F 11 and XeF 5 Ta 2 F 11 , each [A 2 F 11 ] − anion (A = Nb, Ta) participates in four secondary contacts with three different A 2 F 11 dimers (Figures 12-14). In all three salts, the Xe-F ax bonds are shorter than the other four Xe-F eq distances ( Table 5). The A-F b -A bridge (Table 5) [64]. In XeF 5 Sb 2 F 11 , the four F eq atoms of the Sb(1)F 6 unit are in staggered position in respect to the four F eq of the Sb(2)F 6 group with a torsion angle of~37.5 o , while in the corresponding Nb and Ta salts they are almost in an eclipsed position.

Crystal Structure of XeF 5 Ni(AsF 6 ) 3
The crystal structure of XeF 5 Ni(AsF 6 ) 3 ( Figure 15) is isotypical to the crystal structure of XeF 5 Ni(SbF 6 ) 3 [35]. The cation Ni 2+ is coordinated by six fluorine atoms provided by six octahedral anions [AsF 6 ] − forming almost regular NiF 6 octahedra. The Ni−F bond lengths in both salts are virtually identical (Table 6). They range from 1.989 (1) to 2.013 (1) Å. Due to the sharing of fluorine atoms, the NiF 6 and AsF 6 octahedra are connected to form a three-dimensional framework. The [XeF 5 ] + cations are located inside the cavities. The geometry of the [XeF 5 ] + cations is almost identical in both Ni salts ( Table 5).
The crystal structure of XeF5Ni(AsF6)3 ( Figure 15) is isotypical to the crystal structure of XeF5Ni(SbF6)3 [35]. The cation Ni 2+ is coordinated by six fluorine atoms provided by six octahedral anions [AsF6] − forming almost regular NiF6 octahedra. The Ni−F bond lengths in both salts are virtually identical (Table 6). They range from 1.989 (1) to 2.013 (1) Å. Due to the sharing of fluorine atoms, the NiF6 and AsF6 octahedra are connected to form a three-dimensional framework. The [XeF5] + cations are located inside the cavities. The geometry of the [XeF5] + cations is almost identical in both Ni salts (Table 5).

Crystal Structures of the Salts (Xe2F11)2(NiF6)2 and Ni(XeF2)2(IrF6)2
The crystal structure of (Xe2F11)2(NiF6)2 determined at 150 K is the same as that at room temperature [40], which means that there is no phase transition in the 150-296 K range.
It has been reported that the reaction between M n+ (AF6)n − and XeF2 in anhydrous aHF (aHF) leads to coordination compounds [M n+ (XeF2)p](AF6)n − (where XeF2 is coordinated to a metal cation M n+ ) only when the Lewis acidity of M n+ is not high enough to withdraw Fions from XeF2 to form MFn and Xe2F3 + AF6 - [65]. Since the reaction between Ni(AsF6)2 and XeF2 in aHF gave NiF2 and Xe2F3AsF6, the preparation of [Ni(XeF2)2](IrF6)2 was a small surprise. The

Crystal
Structures of the Salts (Xe 2 F 11 ) 2 (NiF 6 ) 2 and Ni(XeF 2 ) 2 (IrF 6 ) 2 The crystal structure of (Xe 2 F 11 ) 2 (NiF 6 ) 2 determined at 150 K is the same as that at room temperature [40], which means that there is no phase transition in the 150-296 K range.
It has been reported that the reaction between M n+ (AF 6 ) n − and XeF 2 in anhydrous aHF (aHF) leads to coordination compounds [M n+ (XeF 2 ) p ](AF 6 ) n − (where XeF 2 is coordinated to a metal cation M n+ ) only when the Lewis acidity of M n+ is not high enough to withdraw F − ions from XeF 2 to form MF n and Xe 2 F 3 + AF 6 − [65]. Since the reaction between Ni(AsF 6 ) 2 and XeF 2 in aHF gave NiF 2 and Xe 2 F 3 AsF 6 , the preparation of [Ni(XeF 2 ) 2 ](IrF 6 ) 2 was a small surprise.
The  (Table 6). They range from 1.989 (1) to 2.013 (1) Å. Due to the sharing of fluorine atoms, the NiF6 and AsF6 octahedra are connected to form a three-dimensional framework. The [XeF5] + cations are located inside the cavities. The geometry of the [XeF5] + cations is almost identical in both Ni salts (Table 5).

Crystal Structures of the Salts (Xe2F11)2(NiF6)2 and Ni(XeF2)2(IrF6)2
The crystal structure of (Xe2F11)2(NiF6)2 determined at 150 K is the same as that at room temperature [40], which means that there is no phase transition in the 150-296 K range.
It has been reported that the reaction between M n+ (AF6)n − and XeF2 in anhydrous aHF (aHF) leads to coordination compounds [M n+ (XeF2)p](AF6)n − (where XeF2 is coordinated to a metal cation M n+ ) only when the Lewis acidity of M n+ is not high enough to withdraw Fions from XeF2 to form MFn and Xe2F3 + AF6 - [65]. Since the reaction between Ni(AsF6)2 and XeF2 in aHF gave NiF2 and Xe2F3AsF6, the preparation of [Ni(XeF2)2](IrF6)2 was a small surprise. The

Materials and Methods
CAUTION: Anhydrous HF and some fluorides are highly toxic and must be handled under a well-ventilated hood, and protective clothing must be worn at all times!
Raman spectra were recorded at room temperature using a Renishaw Raman Imaging Microscope System 1000 or a Horiba Jobin Yvon LabRam-HR spectrometer [66]. The solid starting reagents were loaded into reaction vessels in a dry box (Table S1). The solvent HF and optionally BF 3 , AsF 5 , and SbF 5 were condensed at 77 K to solid reagents, and the reaction vessel was warmed to ambient temperature. Fluorine was slowly added to the reaction vessel at room temperature. A medium-pressure mercury lamp (Hg arc lamp, 450 W, Ace Glass, Vineland, NJ, USA) was used as the UV source. After several days of intensive stirring at room temperature, the volatiles were pumped off and the Raman spectra of the obtained solids were recorded (Figures S1-S11).

Attempted Preparation of XeF
For crystallization, the clear supernatant, which contained no visible sediment, was decanted into the side arm of the crystallization vessel, which consisted of two tubes made of fluoropolymer. Evaporation of the solvent from the side arm was achieved by maintaining a temperature gradient of about 10-20 • C between the two tubes for several weeks. Slow distillation of aHF resulted in crystal growth.
Crystals were immersed in perfluorodecalin (melting point 263 K) in a dry box, selected under the microscope, and mounted on the goniometer head of the diffractometer in a cold nitrogen stream (265-273 K). Some of them were sealed in quartz capillaries used to record Raman spectra at several random positions (Figures S1-S11).

Crystal Structure Determination
Single-crystal X-ray diffraction data of reported crystal structures were acquired at 150 K (for XeF 5 IrF 6 also at 285 K) with a Gemini A diffractometer equipped with an Atlas CCD detector using graphite monochromated MoKα radiation. The data were processed using the CrysAlisPro software suite program package [71]. Analytical absorption corrections were applied to all data sets. All structures were solved using the dual-space algorithm of the program SHELXT [72] implemented in the Olex crystallographic software [73]. Structure refinement for all structures was performed using the software SHELXL-2014 [74]. The crystals of the Ni(XeF 2 ) 2 (IrF 6 ) 2 salt showed reproducible pseudo-merohedral twinning. This problem was solved at the data processing stage, and final refinement was performed using reflections from the main domain. The figures were created using the software Balls and Sticks [75]. The compound XeF 5 Nb 2 F 11 crystallizes in the acentric space group P2 1 .
The very-close-to-zero value of the Flack's parameter (−0.031 (11)) confirms the correctness of the absolute structure.
The supplementary crystallographic data for this work are provided free of charge by the joint Cambridge Crystallographic Data Centre and the Fachinformationszentrum Karlsruhe Access Structures service www. ccdc