X-ray crystal structures of α-KrF2, [KrF][MF6] (M = As, Sb, Bi), [Kr2F3][SbF6]·KrF2, [Kr2F3]2[SbF6] 2·KrF2, and [Kr2F3][AsF6]·[KrF][AsF6];

3002

Inorg. Chem. 2001, 40, 3002-3017

X-ray Crystal Structures of r-KrF₂, [KrF][MF₆] (M ) As, Sb, Bi), [Kr₂F₃][SbF₆]‚KrF₂,

[Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃][AsF₆]‚[KrF][AsF₆]; Synthesis and Characterization of

[Kr₂F₃][PF₆]‚nKrF₂; and Theoretical Studies of KrF₂, KrF⁺, Kr₂F₃, and the [KrF][MF₆]

+

(M ) P, As, Sb, Bi) Ion Pairs^†

John F. Lehmann,^‡David A. Dixon,^§and Gary J. Schrobilgen*^,‡

Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada, and

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National

Laboratory, 906 Batelle Boulevard, P.O. Box 999, KI-83, Richland, Washington 99352

ReceiVed October 23, 2000

The crystal structures of R-KrF₂and salts containing the KrF⁺and Kr₂F₃⁺cations have been investigated for the

first time using low-temperature single-crystal X-ray diffraction. The low-temperature R-phase of KrF₂crystallizes

in the tetragonal space group I4/mmm with a ) 4.1790(6) Å, c ) 6.489(1) Å, Z ) 2, V ) 113.32(3) Å³, R₁)

0.0231, and wR₂) 0.0534 at -125 °C. The [KrF][MF₆] (M ) As, Sb, Bi) salts are isomorphous and isostructural

and crystallize in the monoclinic space group P2₁/c with Z ) 4. The unit cell parameters are as follows: â-[KrF]-

[AsF₆], a ) 5.1753(2) Å, b ) 10.2019(7) Å, c ) 10.5763(8) Å, â ) 95.298(2)°, V ) 556.02(6) Å³, R₁) 0.0265,

and wR₂) 0.0652 at -120 °C; [KrF][SbF₆], a ) 5.2922(6) Å, b ) 10.444(1) Å, c ) 10.796(1) Å, â ) 94.693-

(4)°, V ) 594.73(1) Å³, R₁) 0.0266, wR₂) 0.0526 at -113 °C; [KrF][BiF₆], a ) 5.336(1) Å, b ) 10.513(2)

Å, c ) 11.046(2) Å, â ) 94.79(3)°, V ) 617.6(2) Å³, R₁) 0.0344, and wR₂) 0.0912 at -130 °C. The Kr₂F₃

+

cation was investigated in [Kr₂F₃][SbF₆]‚KrF₂, [Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃][AsF₆]‚[KrF][AsF₆]. [Kr₂F₃]₂-

[SbF₆]₂‚KrF₂crystallizes in the monoclinic P2₁/c space group with Z ) 4 and a ) 8.042(2) Å, b ) 30.815(6) Å,

c ) 8.137(2) Å, â ) 111.945(2)°, V ) 1870.1(7) Å³, R₁) 0.0376, and wR₂) 0.0742 at -125 °C. [Kr₂F₃]-

[SbF₆]‚KrF₂crystallizes in the triclinic P1h space group with Z ) 2 and a ) 8.032(3) Å, b ) 8.559(4) Å, c )

8.948(4) Å, R ) 69.659(9)°, â ) 63.75(1)°, γ ) 82.60(1)°, V ) 517.1(4) Å³, R₁) 0.0402, and wR₂) 0.1039

at -113 °C. [Kr₂F₃][AsF₆]‚[KrF][AsF₆] crystallizes in the monoclinic space group P2₁/c with Z ) 4 and a )

6.247(1) Å, b ) 24.705(4) Å, c ) 8.8616(6) Å, â ) 90.304(6)°, V ) 1367.6(3) Å³, R₁) 0.0471 and wR₂)

0.0958 at -120 °C. The terminal Kr-F bond lengths of KrF⁺and Kr₂F₃are very similar, exhibiting no

+

crystallographically significant variation in the structures investigated (range, 1.765(3)-1.774(6) Å and 1.780-

(7)-1.805(5) Å, respectively). The Kr-F bridge bond lengths are significantly longer, with values ranging from

2.089(6) to 2.140(3) Å in the KrF⁺salts and from 2.027(5) to 2.065(4) Å in the Kr₂F₃salts. The Kr-F bond

+

lengths of KrF₂in [Kr₂F₃][SbF₆]‚KrF₂and [Kr₂F₃]₂[SbF₆]₂‚KrF₂range from 1.868(4) to 1.888(4) Å and are similar

to those observed in R-KrF₂(1.894(5) Å). The synthesis and Raman spectrum of the new salt, [Kr₂F₃][PF₆]‚

nKrF₂, are also reported. Electron structure calculations at the Hartree-Fock and local density-functional theory

levels were used to calculate the gas-phase geometries, charges, Mayer bond orders, and Mayer valencies of

KrF⁺, KrF₂, Kr₂F₃⁺, and the ion pairs, [KrF][MF₆] (M ) P, As, Sb, Bi), and to assign their experimental vibrational

frequencies.

Introduction

and KrF₂‚MnF₄,²²the RCN-KrF⁺(R ) H, CF₃, C₂F₅, n-C₃F₇)

cations,^23,24and Kr(OTeF₅)₂.²⁵While the strong oxidant char-

acters of KrF₂, KrF⁺, and Kr₂F₃⁺provide clean, low-temperature

The known compounds of krypton are limited to the +2

oxidation state and include KrF₂,^1-9salts of the KrF⁺,^10-18and

+ 13,26

+ 27

synthetic routes to BrF₆

,

ClF₆

,

OsO₂F₄,²⁸AuF₅,¹⁴and

+ 11-13,15,16,18

Kr₂F₃

cations, the molecular adducts KrF₂‚MOF₄

TcOF₅,²⁹they have also served as a significant impediment to

their detailed structural characterization by single-

(M ) Cr, Mo, W),^19,20KrF₂‚nMoOF₄(n ) 2, 3),¹⁹KrF₂‚VF₅,²¹

^†Dedicated to the memory of our colleague and friend, Karel (Drago)

Lutar (December 1, 1947-September 2, 2000) in recognition of his many

outstanding contributions to the field of inorganic fluorine chemistry.

* To whom correspondence should be addressed.

^‡McMaster University.

(7) (a) Bezmel’nitsyn, V. N.; Legasov, V. A.; Chaivanov, B. B. Proc.

Acad. Sci. USSR, Chem. Sect. 1977, 235, 365. (b) Dokl. Akad. Nauk.

SSSR 1977, 235, 96.

(8) Sˇmalc, A.; Lutar, K.; Zˇemva, B. Inorg. Synth. 1992, 29, 11.

(9) Kinkead, S. A.; FitzPatrick, J. R.; Foropoulos, J., Jr.; Kissane, R. J.;

Purson, J. D. In Fluorine Chemistry Toward the 21st Century;

Thrasher, J. S., Strauss, S. H., Eds.; ACS Symposium Series 555;

American Chemical Society: Washington, DC, 1994; Chapter 3, pp

40-55.

(10) Selig, H.; Peacock, R. D. J. Am. Chem. Soc. 1964, 86, 5.

(11) Frlec, B.; Holloway, J. H. J. Chem. Soc., Chem. Commun. 1973, 370.

(12) Frlec, B.; Holloway, J. H. J. Chem. Soc., Chem. Commun. 1974, 89.

(13) Gillespie, R. J.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.

1974, 90.

^§Pacific Northwest National Laboratory.

(1) Turner, J. J.; Pimental, G. C. Science 1963, 140, 974.

(2) MacKenzie, D. R. Science 1963, 141, 1171.

(3) Schreiner, F.; Malm, J. G.; Hindman, J. C. J. Am. Chem. Soc. 1965,

87, 25.

(4) Streng, L. V.; Streng, A. G. Inorg. Chem. 1966, 5, 328.

(5) Prusakov, V. N.; Sokolov, V. B. At. Energy 1971, 31, 259.

(6) Slivnik, J.; Sˇmalc, A.; Lutar, K.; Zˇemva, B.; Frlec, B. J. Fluorine

Chem. 1975, 5, 273.

Published on Web 05/22/2001

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3003

+

crystal X-ray diffraction. With the exception of KrF₂, no other

krypton compound has been structurally characterized in detail

by diffraction techniques. Of the two phases of KrF₂identified

by variable-temperature Raman spectroscopy,³⁰only the X-ray

crystal structure of the high-temperature â-phase has been

reported at -80 °C,³¹although an incorrect determination of

the unit cell parameters of the â-phase at room temperature had

been reported earlier.³²

Kr₂F₃salts have been assigned on the basis of a V-shaped

fluorine-bridged geometry for the cation,^13,16analogous to that

established by X-ray crystallography for Xe₂F₃.

^{+ 37,38}Unlike the

vibrational assignments for Xe₂F₃

,

^{+ 37,38}the vibrational assign-

ments of Kr₂F₃are based on an unsymmetrical cation.^13,15,16

In contrast with the KrF⁺salts, the cation-anion interactions

in the Kr₂F₃⁺salts appear to be weak, as indicated by retention

of octahedral symmetry by the anion. The Lewis acid properties

of KrF⁺that lead to fluorine bridging in the salts are further

manifested in the formation of adducts with the nitrogen electron

pair in oxidatively resistant nitriles to give the adduct cations

RCN-KrF⁺(R ) H, CF₃, C₂F₅, n-C₃F₇), which provide the

first examples of Kr-N bonds.^23,24Fluorine-19 NMR spectros-

copy has also been used to study the KrF⁺cation in HF solution

+

Krypton difluoride has been shown to exhibit fluoride ion

donor properties that are analogous to those established for

XeF₂.^18,19,33-36The reaction of KrF₂with strong fluoride ion

+

acceptors leads to the formation of a number of KrF⁺and Kr₂F₃

salts, i.e., [KrF][MF₆] (M ) As, Sb, Bi, Au, Pt, Ta),^11-17[KrF]-

[M₂F₁₁] (M ) Sb, Ta, Nb),^{10,11,13,15,16}and [Kr₂F₃][MF₆] (M )

As, Sb, Ta).^11-13,15,16The physical and spectroscopic properties

of these salts have been reviewed by Selig and Holloway.¹⁸

Unlike their xenon(II) analogues, the majority of, if not all,

krypton(II) compounds are thermodynamically unstable. The

relative kinetic stabilities of their salts, however, show consider-

able variance.^11-17A number of salts such as [KrF][SbF₆],^12,13,16

[KrF][Sb₂F₁₁],^10,15,16[Kr₂F₃][SbF₆],^13,15,16and [KrF][AuF₆]¹⁴can

be handled and stored at room temperature for appreciable

periods of time without significant decomposition.

+

and the Kr₂F₃cation in BrF₅solution and has provided the

first unambiguous characterization of the structure of the

fluorine-bridged Kr₂F₃cation.^13,16

+

The adducts of KrF₂with weak fluoride ion acceptor metal

oxide tetrafluorides, KrF₂‚MOF₄(M ) Mo, W) and KrF₂‚

nMoOF₄(n ) 2, 3), have been extensively studied in SO₂ClF

solution by low-temperature ¹⁹F NMR spectroscopy and have

been shown to contain Kr-F_b--M (F_b, bridge fluorine) bridges,

which are nonlabile in solution at low temperatures on the NMR

time scale.¹⁹The adducts KrF₂‚MOF₄(M ) Cr, Mo, W)^19,20

and KrF₂‚VF₅

+

Structural characterization of the KrF⁺and Kr₂F₃salts in

21

have been studied in the solid state by

the solid state using Raman spectroscopy has been most

extensive.^11-17The Raman spectra of KrF⁺salts also indicate

that the KrF⁺cation strongly interacts with the anion by

formation of a fluorine bridge between krypton and a fluorine

of the anion, as is the case for XeF⁺in the crystal structures of

[XeF][AsF₆],³⁵[XeF][Sb₂F₁₁],³³and [XeF][RuF₆].³⁴Conse-

quently, fluorine bridge modes and vibrational modes resulting

from symmetry lowering of the octahedral anion have been

reported and tentatively assigned.^14-17The Raman spectra of

vibrational spectroscopy. The compounds are consistent with

the weaker fluoride ion acceptor properties of MOF₄and VF₅,

and are best represented as covalent adducts in which KrF₂is

weakly fluorine bridged to the transition metal, so that the Kr-

F_t(F_t, terminal fluorine) and Kr-F_bbond lengths are expected

to be similar to those of free KrF₂. With the exception of XeF₂‚

CrOF₄, which is presently unknown, the vibrational spectra¹⁹

and solution ¹⁹F and ¹²⁹Xe NMR spectra³⁶of the analogous XeF₂

adducts of MoOF₄and WOF₄have been reported, and the crystal

structure of F-Xe-F--WOF₄has been determined.³⁹

The present study reports the first detailed X-ray crystal-

lographic study of salts containing the KrF⁺and Kr₂F₃⁺cations

and confirms the dimorphism of KrF₂by determination of the

X-ray structure of the low-temperature phase. Theoretical

calculations have been used to arrive at the energy-minimized

gas-phase geometries and to reassign or confirm earlier vibra-

tional assignments of KrF₂, the [KrF][MF₆] (M ) P, As, Sb,

Bi) ion pairs, and Kr₂F₃⁺in the gas phase. The effects of crystal

packing on the geometries of the [KrF][MF₆] ion pairs and the

Kr₂F₃⁺cation are also assessed in light of their calculated gas-

phase geometries.

(14) Holloway, J. H.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.

1975, 623.

(15) Frlec, B.; Holloway, J. H. Inorg. Chem. 1976, 15, 1263.

(16) Gillespie, R. J.; Schrobilgen, G. J. Inorg. Chem. 1976, 15, 22.

(17) Gillespie, R. J.; Martin, D.; Schrobilgen, G. J. J. Chem. Soc., Dalton

Trans. 1980, 1898.

(18) Selig, H.; Holloway, J. H. In Topics in Current Chemistry; Boschke,

F. L., Ed.; Springer-Verlag: Berlin, 1984; Vol. 124, pp 33-90.

(19) Holloway, J. H.; Schrobilgen, G. J. Inorg. Chem. 1981, 20, 3363.

(20) Christe, K. O.; Wilson, W. W.; Bougon, R. A. Inorg. Chem. 1986,

25, 2163.

(21) Zˇemva, B.; Slivnik, J.; Sˇmalc, A. J. Fluorine Chem. 1975, 6, 191.

(22) Lutar, K.; Jesih, A.; Zˇemva, B. Polyhedron 1988, 7, 1217.

(23) Schrobilgen, G. J. J. Chem. Soc., Chem. Commun. 1988, 863.

(24) Schrobilgen, G. J. J. Chem. Soc., Chem. Commun. 1988, 1506.

(25) Sanders, J. C. P.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.

1989, 1576.

(26) Gillespie, R. J.; Schrobilgen, G. J. Inorg. Chem. 1974, 13, 1230.

(27) Christe, K. O.; Wilson, W. W.; Curtis, E. C. Inorg. Chem. 1983, 22,

3056.

(28) Christe, K. O.; Dixon, D. A.; Mack, H. G.; Oberhammer, H.; Pagelot,

A.; Sanders, J. C. P.; Schrobilgen, G. J. J. Am. Chem. Soc. 1993, 115,

11279.

(29) LeBlond, N.; Mercier, H. P. A.; Dixon, D. A.; Schrobilgen, G. J. Inorg.

Chem. 2000, 39, 4494.

(30) Al-Mukhtar, M.; Holloway, J. H.; Hope, E. G.; Schrobilgen, G. J. J.

Chem. Soc., Dalton Trans. 1991, 2831.

(31) Burbank, R. D.; Falconer, W. E.; Sunder, W. A. Science 1972, 178,

1285.

(32) Siegel, S.; Gebert, E. J. Am. Chem. Soc. 1964, 86, 3896.

(33) Burgess, J.; Fraser, C. J. W.; McRae, V. M.; Peacock, R. D.; Russell,

D. R. In Herbert H. Hyman Memorial Volume, J. Inorg. Nucl. Chem.

Supplement; Katz, J. J., Sheft, I., Eds.; Pergamon Press: Oxford, 1976;

p 183.

(34) Bartlett, N.; Gennis, M.; Gibler, D. D.; Morrell, B. K.; Zalkin, A.

Inorg. Chem. 1973, 12, 1717.

(35) Zalkin, A.; Ward, D. L.; Biagioni, R. N.; Templeton, D. H.; Bartlett,

N. Inorg. Chem. 1978, 17, 1318.

Results and Discussion

X-ray Crystal Structures of r-KrF₂and Salts of the KrF⁺

and Kr₂F₃⁺Cations. The unit cell parameters and refinement

statistics for the title compounds are given in Table 1. Important

bond lengths, contacts, and bond angles are listed in Tables 2-4.

With the exception of [Kr₂F₃][AsF₆]‚[KrF][AsF₆], which was

+

refined as a merohedral twin, all of the KrF⁺and Kr₂F₃salts

were isolated and solved as single crystals.

The [KrF][MF₆] (M ) As, Sb, Bi), [Kr₂F₃][SbF₆]‚KrF₂,

[Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃][AsF₆]‚[KrF][AsF₆] salts were

synthesized by reaction of KrF₂with MF₅in 1:1, 1:1, 2:1, 3.5:

(37) Bartlett, N.; DeBoer, B. G.; Hollander, F. J.; Sladky, F. O.; Templeton,

D. H.; Zalkin, A. Inorg. Chem. 1974, 13, 780.

(38) Fir, B. A.; Gerken, M.; Pointner, B. E.; Mercier, H. P. A.; Dixon, D.

A.; Schrobilgen, G. J. J. Fluorine Chem. 2000, 105, 159.

(39) Tucker, P. A.; Taylor, P. A.; Holloway, J. H.; Russell, D. R. Acta

Crystallogr., Sect. B 1975, 31, 906.

(36) Holloway, J. H.; Schrobilgen, G. J. Inorg. Chem. 1980, 19, 2632.

3004 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

Table 1. Summary of Crystal Data and Refinement Results for R-KrF₂, [KrF][MF₆] (M ) As, Sb, Bi), [Kr₂F₃][SbF₆]‚KrF₂,

[Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃][AsF₆]‚[KrF][AsF₆]

[Kr₂F₃]₂[SbF₆]₂‚ [Kr₂F₃][SbF₆]‚ [Kr₂F₃][AsF₆]‚

R-KrF₂

I4/mmm

â-[KrF][AsF₆]

[KrF][SbF₆]

[KrF][BiF₆]

KrF₂

[KrF][AsF₆]

space group

a (Å)

b (Å)

P2₁/c

P1h

P2₁/c

4.1790(6)

6.489(1)

90

113.32(3)

2

121.80

3.570

-125

19.60

5.1753(2)

10.2019(7)

10.5763(8)

90

95.298(2)

90

556.02(6)

4

291.72

3.485

5.2922(6)

10.444(1)

10.796(1)

90

94.693(4)

90

594.73(1)

4

338.55

3.781

5.336(1)

10.513(2)

11.046(2)

90

94.79(3)

90

617.6(2)

4

425.78

4.579

8.042(2)

30.815(6)

8.137(2)

90

111.945(3)

90

1870.1(7)

4

1042.47

3.703

8.032(3)

8.559(4)

8.948(4)

69.659(9)

63.75(1)

82.60(1)

517.1(4)

2

582.15

3.739

-113

15.54

6.247(1)

24.705(4)

8.8616(6)

90

90.304(6)

90

1367.6(3)

4

705.24

3.425

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

Z (molecules/unit cell)

mol wt (g mol^-1

)

F

_calcd(g cm^-3

)

T (°C)

-120

14.09

R₁) 0.0265

-113

12.09

R₁) 0.0266

-130

35.72

R₁) 0.0344

-125

14.82

R₁) 0.0376

-120

14.70

R₁) 0.0471

µ (mm^-1

)

final agreement

R₁) 0.0231

R₁) 0.0402

wR₂) 0.1039 wR₂) 0.0958

factors^a,b

wR₂) 0.0534 wR₂) 0.0652 wR₂) 0.0526 wR₂) 0.0912 wR₂) 0.0742

^aR₁) ∑||F_o| - |F_c||/∑|F_o| for I > 2σ(I). ^bwR₂) ∑|(|F_o| - |F_c|)w^1/2|/∑(|F_o|w) for I > 2σ(I).

Table 2. Experimental and Calculated Bond Lengths (Å) and Bond

Angles (deg) for KrF₂

parallel to each other and to the c-axis, with eight molecules

centered at the corners of the unit cell and one located in the

center of the unit cell. This contrasts with the â-phase in which

the KrF₂molecules are centered on the corners of the unit cell

lying in the ab-plane are rotated by 45° with respect to the

a-axis. The central KrF₂molecule also lies in the ab-plane but

is perpendicular with respect to the molecular axes of KrF₂

molecules occupying the corner positions.³¹Interconversion of

the â- and R-phases occurs without loss of crystallinity by means

of a 45° rotation of the molecules occupying the corner sites,

such that they align with the a-axis of the â-phase and a 45°

rotation of the central molecule in the opposite direction such

that it also lies parallel to the a-axis of the R-phase. As a result,

the unit cell axes of the â-phase transform into those of the

R-phase as follows: a f c and b, c f a, b or b, a.

The Kr-F bond length in R-KrF₂is 1.894(5) Å (Table 2)

and is in excellent agreement with those determined for â-KrF₂

(1.89(2) Å, -80 °C) by X-ray diffraction³¹and for gaseous KrF₂

by electron diffraction (1.889(10) Å, -40 °C).⁴¹As observed

for â-KrF₂, R-KrF₂is linear by symmetry. Despite contraction

of the unit cell volume for the R-phase (113.32(3) Å³, -125

°C) relative to that of the â-phase (122.5 Å³, -80 °C) and

reorientation of KrF₂molecules within the cell, the interatomic

F‚‚‚F distance between collinearly orientated KrF₂molecules

is 2.71 Å in both structures.

â-[KrF][AsF₆], [KrF][SbF₆], and [KrF][BiF₆]. The title

compounds form an isomorphous and isostructural series (Figure

2) in which the KrF⁺cation strongly interacts with the anion

by forming a fluorine bridge with the pseudo-octahedral anion

that is bent about F_b. The [KrF][MF₆] ion pairs pack in columns

along the a-axis of the unit cell and are orientated such that

alternating cation and anion planes are parallel to the b-c face

(Figure 2d). The point group symmetries of the [KrF][MF₆] ion

pairs are C₁because the F_b--Kr-F_tgroup is skewed with respect

to a plane defined by the axial fluorine (F_a) trans to the bridging

fluorine (F_b) and two equatorial fluorines (F_e) trans to each other.

The smallest F_e-M- -F_b- -Kr dihedral angles are 22.1, 22.2, 19.7,

and 28.1° for â-[KrF][AsF₆], [KrF][SbF₆], [KrF][BiF₆], and

[Kr₂F₃][AsF₆]‚[KrF][AsF₆], respectively.

a

b

R-KrF₂

â-KrF₂

KrF₂(gas)^c

LDFT

1.910

HF

Bond Lengths (Å)

1.89(2)

2.71

Kr-F

F- - -F

1.894(5)

2.71

1.889(10)

1.822

Bond Angles (deg)

F-Kr-F

180

^aThis work. ^bFrom ref 31. ^cFrom ref 41.

1, and 2:1 molar ratios, respectively, in anhydrous HF solvent.

Despite repeated attempts, crystals of [Kr₂F₃][SbF₆] could not

be obtained. The stoichiometric mixture, 2KrF₂:SbF₅, was found

to decompose rapidly in HF at temperatures as low as -40 °C,

resulting in the growth of crystals of the less soluble [KrF][SbF₆]

salt. Attempts to crystallize [Kr₂F₃][SbF₆] using an excess of

KrF₂to account for the decomposition resulted in a mixture of

crystalline [KrF][SbF₆] and [Kr₂F₃]₂[SbF₆]₂‚KrF₂or [Kr₂F₃]-

[SbF₆]‚KrF₂. Crystallization of [Kr₂F₃][AsF₆] was also unsuc-

cessful but resulted in the discovery of the new crystalline

double salt, [Kr₂F₃][AsF₆]‚[KrF][AsF₆].

r-KrF₂. The crystal structure of R-KrF₂(Figure 1) was

determined in order to obtain more precise structural parameters

for KrF₂, which are important for comparison with other Kr-F

bond lengths, and to establish the structural basis for the

dimorphism of KrF₂. The dimorphism of KrF₂was originally

studied by Raman spectroscopy in conjunction with a factor-

group analysis. This study indicated that the low-temperature

phase, R-KrF₂, is isomorphous with XeF₂, which crystallizes

in the tetragonal space group I4/mmm at room temperature.⁴⁰

The present study confirms the dimorphism of KrF₂, the

correctness of the earlier vibrational spectroscopic analysis, and

the space group assignment. The crystal structure of â-KrF₂has

been previously determined at -80 °C, and the â f R phase

transition has been shown by variable-temperature Raman

spectroscopy to occur below this temperature.³⁰The fact that

crystals of â-KrF₂were initially grown by sublimation at -78

°C in the present study and then cooled to -125 °C for X-ray

data collection on the R-phase indicates that the âfR phase

transition occurs without serious disruption of the crystal lattice

(vide infra).

As inferred from the Raman spectra, the Kr-F_tbond lengths

(1.765(3)-1.774(6) Å) in these salts are significantly shorter,

and the Kr- -F_bbridge bond lengths are significantly longer than

the Kr-F bonds of R-KrF₂(1.894(5) Å). The MF₆^-octahedra

The crystal packing of R-KrF₂(Figure 1) is analogous to that

of XeF₂.⁴⁰All the KrF₂molecules in this phase are arranged

(40) Agron, P. A.; Begun, G. M.; Levy, H. A.; Mason, A. A.; Jones, C.

(41) Harshberger, W.; Bohn, R. K.; Bauer, S. H. J. Am. Chem. Soc. 1967,

89, 6466.

G.; Smith, D. F. Science 1963, 139, 842.

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3005

Table 3. Experimental and Calculated Bond Lengths (Å) and Bond Angles (deg) in [KrF][MF₆] (M ) As, Sb, Bi) and Calculated Bond

Lengths (Å) and Bond Angles (deg) in [KrF][MF₆]

[Kr₂F₃][AsF₆]‚

â-[KrF][AsF₆]

[KrF][AsF₆]^a

exp

[KrF][SbF₆]

LDFT

[KrF][BiF₆]

LDFT

[KrF][PF₆]

LDFT HF

exp

LDFT

HF

exp

HF

exp

HF

Bond Lengths (Å)

Kr-F(1)

Kr- - -F(2)

M- -F(2)

M-F(3)

M-F(4)

M-F(5)

M-F(6)

M-F(7)

1.765(2)

2.131(2)

1.845(2)

1.691(2)

1.698(2)

1.710(2)

1.703(2)

1.867

1.998

2.096

1.709

1.729

1.752

1.734

1.746

2.002

1.972

1.659

1.672

1.700

1.681

1.783(6)

2.106(6)

1.878(6)

1.686(6)

1.711(6)

1.684(7)

1.720(6)

1.685(7)

1.765(3)

2.140(3)

1.963(3)

1.856(3)

1.847(3)

1.859(3)

1.861(3)

1.857(3)

1.857

1.739

2.038

2.049

1.813

1.821

1.841

1.774(6)

2.090(6)

2.106(6)

1.944(6)

1.957(6)

1.962(6)

1.938(6)

1.941(6)

1.859

2.012

2.266

1.948

1.977

1.957

1.745

2.003

2.194

1.950

1.969

1.958

1.877

1.972

2.176

1.561

1.588

1.609

1.592

1.789

1.873

2.493

1.526

1.533

1.572

1.556

2.017

2.185

1.898

1.907

1.925

Bond Angles (deg)

F(1)-Kr- -F(2) 176.8(1)

Kr- -F(2)-M 133.7(1)

177.7

120.0

178.3

128.3

177.3(3)

124.6(3)

177.9(2)

139.2(2)

177.4

114.1

178.6

124.9

177.0(4)

138.3(3)

177.2

113.8

178.7

131.2

178.3

119.1

179.3

117.2

^aThe fluorine atom labels F(4), F(5), F(6), F(7), F(8), F(9), and F(10) for the [KrF][AsF₆] unit in [Kr₂F₃][AsF₆]‚[KrF][AsF₆] (see Figure 5) are

given as F(1), F(2), F(3), F(4), F(5), F(6), and F(7), respectively, in this table.

+

-

Table 4. Experimental Bond Lengths (Å) and Bond Angles (deg) for the Kr₂F₃Cation and MF₆(M ) As, Sb) Anion in [Kr₂F₃][SbF₆]‚KrF₂,

+

[Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃][AsF₆]‚[KrF][AsF₆] and Calculated Bond Lengths (Å) and Bond Angles (deg) in Kr₂F₃

+

exp

Kr₂F₃

LDFT (C_2V)

[Kr₂F₃]₂[SbF₆]₂‚KrF₂

[Kr₂F₃][SbF₆]‚KrF₂

[Kr₂F₃][AsF₆]‚[KrF][AsF₆]

HF (D_∞h

)

Bond Lengths (Å)

Kr(1)-F(1)

Kr(2)-F(2)

Kr(1)- - -F(3)

Kr(2)- - -F(3)

Kr(3)-F(4)

Kr(4)-F(5)

Kr(3)- - -F(6)

Kr(4)- - -F(6)

Kr(5)-F(7)

Kr(5)-F(8)

M-F(9)

1.805(5)

1.799(4)

2.041(4)

2.065(4)

1.797(5)

1.787(4)

2.052(5)

2.056(4)

1.881(4)

1.887(4)

1.872(4)

1.880(4)

1.876(4)

1.879(4)

1.873(4)

1.872(4)

1.882(4)

1.874(4)

1.882(4)

1.876(4)

1.879(4)

1.872(4)

1.876(4)

1.800(5)

1.790(5)

2.027(5)

2.046(5)

1.780(7)

1.803(6)

2.061(6)

2.049(6)

1.826

2.081

1.730

2.082

1.868(4)

1.888(4)

1.875(4)

1.874(4)

1.872(4)

1.861(4)

1.875(4)

1.886(4)

M-F(10)

M-F(11)

1.689(7)

1.701(7)

1.719(7)

1.710(8)

1.674(9)

1.712(6)

M-F(12)

M-F(13)

M-F(14)

M-F(15)

M-F(16)

M-F(17)

M-F(18)

M-F(19)

M-F(20)

M-F₆(mean)

1.874(8)

1.701(17)

Bond Angles (deg)

175.1(2)

176.8(2)

F(1)-Kr(1)- - -F(3)

F(2)-Kr(2)- - -F(3)

F(4)-Kr(3)- - -F(6)

F(5)-Kr(4)- - -F(6)

Kr(1)- - -F(3)- - -Kr(2)

Kr(3)- - -F(6)- - -Kr(4)

F(7)-Kr(5)-F(8)

177.9(2)

177.8(2)

178.7(2)

178.3(2)

126.0(2)

128.0(2)

179.1(2)

178.2(3)

178.6(3)

177.4

180.0

142.5(3)

178.0(2)

127.5(3)

135.2

180.0

are distorted and have local C_4Vsymmetries that result from

strong bridging to the cation, giving rise to a long M- -F_bbond.

There is no clear distinction between the M-F_eand the M-F_a

bond lengths in these compounds. Despite the strong fluorine

bridge interactions and variations in the fluoride ion acceptor

strengths of AsF₅, SbF₅, and BiF₅, the Kr-F_tbond lengths in

all three salts and the Kr- -F_bbond distances in â-[KrF][AsF₆]

(2.131(2) Å) and [KrF][SbF₆] (2.140(3) Å) exhibit no crystal-

lographically significant variation at the 3σ limit. The Kr- -F_b

Comparison of the Xe- -F_bbonds in â-[XeF₃][SbF₆]⁴²and [XeF₃]-

[BiF₆]⁴³reveals the same trend. The Kr- -F_bbond distances of

all three salts are significantly less than the sum of the fluorine

(1.47, 1.35 Å)^44,45and krypton (2.02 Å)⁴⁴van der Waals radii

and are indicative of significant covalent bond character that is

also reflected in the nonlinearity of the Kr- -F_b-M bridge angles

(vide infra).

(42) Boldrini, P.; Gillespie, R. J.; Ireland, P. R.; Schrobilgen, G. J. Inorg.

Chem. 1974, 13, 1690.

(43) Gillespie, R. J.; Martin, D.; Schrobilgen, G. J.; Slim, D. R. J. Chem.

Soc., Dalton Trans. 1977, 2234.

(44) Bondi, A. J. Phys. Chem. 1964, 68, 441.

(45) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell

University Press: Ithaca, NY, 1960; p 260.

-

bridge bond length is, however, significantly shorter in the BiF₆

salt (2.089(6) Å) than in the arsenic and antimony analogues,

which is consistent with a weaker fluoride acceptor strength

for BiF₅when compared with those of AsF₅and SbF₅.

3006 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

predicted by the VSEPR model of molecular geometry,⁴⁹were

observed for the F_t-Kr- -F_bbond angle, which ranged from

176.8(1) to 177.9(2)° in the three KrF⁺salts. Similar deviations

from linearity are also obtained for the energy-minimized gas-

phase geometries (see Computational Results), and therefore

cannot be attributed to packing effects alone. The Kr- -F_b- -M

bridge bond angles, which also exhibit minor variations over

the series â-[KrF][AsF₆] (133.7(1)°), [KrF][SbF₆] (139.2(2)°),

and [KrF][BiF₆] (138.3(3)°), are consistent with the bent

geometry predicted for an AXYE₂(A ) F_b, X ) Kr, Y ) M)

VSEPR arrangement. The angles are considerably more open

than the ideal tetrahedral angle owing to the high ionic character

of the Kr- -F_bbond.

The structure of the [KrF][AsF₆] ion pair in [Kr₂F₃][AsF₆]‚

[KrF][AsF₆] differs from that of â-[KrF][AsF₆] in that the

dihedral F_e-M- -F_b- -Kr angle (28.1°) in the double salt is larger

than that of â-[KrF][AsF₆] (22.1°) and indicates that the

geometry of the ion pair is significantly influenced by the crystal

packing. The Kr- -F_bbond length is significantly shorter (2.106-

(6) Å) than in â-[KrF][AsF₆], while the Kr-F_tbond length

(1.783(6) Å) shows no significant change.

The effects of secondary Kr‚‚‚F contacts on the structures of

[KrF][MF₆] (M ) As, Sb, Bi) and [Kr₂F₃][AsF₆]‚[KrF][AsF₆]

are difficult to assess. Each isomorphous [KrF][MF₆] structure

has eight secondary intermolecular Kr‚‚‚F contacts (2.980-

3.480 Å) that are less than or equal to the sum of the krypton

and fluorine van der Waals radii, while the krypton atom of

KrF⁺in [Kr₂F₃][AsF₆]‚[KrF][AsF₆] has seven such contacts

(3.076-3.489 Å). Intramolecular contacts between krypton and

an equatorial fluorine in the anion of the same ion pair are only

observed in [KrF][AsF₆] (3.301 Å) and [Kr₂F₃][AsF₆]‚[KrF]-

[AsF₆] (3.061, 3.449 Å).

Figure 1. Packing diagram of R-KrF₂along the a-axis.

[Kr₂F₃]₂[SbF₆]₂‚KrF₂, [Kr₂F₃][SbF₆]‚KrF₂, and [Kr₂F₃]-

[AsF₆]‚[KrF][AsF₆]. The X-ray crystal structures of the title

compounds, along with their packing diagrams, are depicted in

Figures 3-5. When viewed along the a-axis, the crystal packing

of [Kr₂F₃][SbF₆]‚KrF₂consists of alternating cation and equally

Variable-temperature Raman spectroscopy has previously

shown that [KrF][AsF₆] exhibits a phase transition between 25

and -78 °C. It was expected that [KrF][SbF₆] and the low-

temperature â-phase of [KrF][AsF₆] would prove to be isomor-

phous and isostructural because their similar Raman spectra

exhibit the same split ν(Kr-F_t) (615, 619 cm^-1) and ν(Kr- -F_b)

(338 cm^-1) stretching frequencies.¹⁶The structural relationship

has now been verified by the crystal structures of these

compounds and their near identical Kr-F_tand Kr- -F_bbond

lengths. Although significant lengthening of the Kr-F_tbond

with respect to the arsenic and antimony compounds is not

observed in the [KrF][BiF₆] structure, the split Kr-F_tstretching

frequency is shifted to lower frequency (604, 610 cm^-1) and is

consistent with a shortening of the Kr- -F_bbond length relative

to those in the arsenic and antimony compounds. The effect on

the Kr- -F_bstretch cannot be directly assessed for [KrF][BiF₆]

because the experimental frequency of this vibration has not

been reported; however, the frequency of this mode has been

calculated (see Computational Results) and is predicted to have

a value similar to that predicted for [KrF][AsF₆]. The splitting

of the Kr-F_tstretch in all three compounds^16,17is now readily

understood in terms of their crystal structures and arises from

vibrational coupling among the four equivalent structural units

in each of their unit cells. A factor-group analysis correlates

the C₁symmetry of the four free ion pairs to their site symmetry,

C₁. Correlation of the C₁site symmetry to the unit cell

symmetry, C_2h, reveals that each vibrational mode is split into

two Raman-active components having A_gand B_gsymmetries

and into two infrared-active components having A_uand B_u

symmetries.^46-48

+

populated anion/KrF₂layers (Figure 3b). Columns of Kr₂F₃

cations run parallel to the a-axis with the cations eclipsing each

-

other when viewed along this axis. Columns of alternating SbF₆

anions and KrF₂molecules are also observed along the a-axis;

however, the KrF₂molecules are off center with respect to the

anions.

The packing of [Kr₂F₃]₂[SbF₆]₂‚KrF₂(Figure 4b) is signifi-

cantly more complex than that of [Kr₂F₃][SbF₆]‚KrF₂. When

viewed along the c-axis, a layered structure is not evident;

however, a complex system of columns can be identified. Half

of the Kr₂F₃⁺cations form columns in which the cations eclipse

one another, while the other half is shifted off center and

alternates with the SbF₆^-anions of an adjacent column, which

also runs parallel to the c-axis. The remaining anions and KrF₂

molecules are arranged in columns in which the KrF₂molecules

are off center with respect to the anion, similar to that observed

in [Kr₂F₃][SbF₆]‚KrF₂. When viewed along the a-axis, there is

also no apparent layering, and the view is similar to that

observed along the c-axis.

Of the solid-state structures investigated in this study, only

(46) Bhagavantam, S.; Venkatarayudu, R. Proc. Indian Acad. Sci. 1939,

9A, 224.

(47) Carter, R. L. J. Chem. Educ. 1971, 48, 297.

(48) Fateley, W. G.; Dollish, F. R.; McDevitt, N. T.; Bentley, F. F. Infrared

and Raman Selection Rules for Molecular and Lattice Vibrations: The

Correlation Method; Wiley: New York, 1972.

Minor deviations from the linear AXYE₃(A ) Kr, X ) F_t,

Y ) F_b, E ) valence electron lone-pair domain) geometry,

(49) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry;

Allyn and Bacon: Boston 1991.

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3007

Figure 2. Structures of (a) [KrF][AsF₆], (b) [KrF][SbF₆], (c) [KrF][BiF₆], and (d) packing diagram viewed along the a-axis showing the head to

tail arrangement of the isomorphous ion pairs, [KrF][MF₆] (M ) As, Sb, Bi); thermal ellipsoids are shown at the 50% probability level.

[Kr₂F₃][AsF₆]‚[KrF][AsF₆] exhibited crystal twinning. Twinning

in this structure is not surprising because of its nearly orthor-

hombic unit cell (â ) 90.304(6)°), which can allow a 90°

reorientation of the crystal growth along the b-axis. The refined

structure was found to contain a 0.489:0.511 population ratio

for the two orientations. When viewed along the a-axis (Figure

5b), eclipsed columns of Kr₂F₃⁺, AsF₆^-, and [KrF][AsF₆] are

observed. Double layers of [KrF][AsF₆] alternate with double

layers of [Kr₂F₃][AsF₆] parallel to the ac-plane.

proposed on the basis of Raman spectroscopic evidence.^13,15,16

In all other cases, Ng₂F₃cations have been described as

+

symmetrical, having C_2Vpoint group symmetries.

+

The F_t-Kr- -F_bbond angles of the Kr₂F₃cation structures

investigated exhibit small, but significant, distortions from the

expected linear AXYE₃VSEPR arrangements, ranging from

177.8(2) to 178.7(2)° in [Kr₂F₃]₂[SbF₆]₂‚KrF₂, from 175.1(2)

to 176.8(2)° in [Kr₂F₃][SbF₆]‚KrF₂, and from 178.2(3) to 178.6-

(3)° in [Kr₂F₃][AsF₆]‚[KrF][AsF₆]. The most significant bond

angle variation occurs for the Kr- -F_b- -Kr bridge, which is

126.0(2) to 128.0(2)° in [Kr₂F₃]₂[SbF₆]₂‚KrF₂, 127.5(3)° in

[Kr₂F₃][AsF₆]‚[KrF][AsF₆], and largest in [Kr₂F₃][SbF₆]‚KrF₂

(142.5(3)°). The variability of the Kr- -F_b- -Kr bond angles of

+

The Kr-F_tbond lengths of the Kr₂F₃cations exhibit no

crystallographically significant variation among the structures

of [Kr₂F₃]₂[SbF₆]₂‚KrF₂(Figure 3a), [Kr₂F₃][SbF₆]‚KrF₂(Figure

4a), and [Kr₂F₃][AsF₆]‚[KrF][AsF₆] (Figure 5a), ranging from

1.780(7) to 1.805(5) Å with an average value of 1.796(9) Å.

The longer and more polar Kr- -F_bbonds are somewhat more

sensitive to their environments and range from 2.027(5) to

2.065(4) Å with an average value of 2.048(14) Å. Long Kr‚‚‚F

interionic contacts are likely responsible for the variability of

Kr- -F_bbond lengths and Kr- -F_b- -Kr bond angles in the

structures that have been studied. Half of the Kr₂F₃⁺cations in

[Kr₂F₃]₂[SbF₆]₂‚KrF₂are asymmetrical and exhibit a small, but

significant, bond length difference between Kr(1)-F_b(3) (2.041-

(4) Å) and Kr(2)-F_b(3) (2.065(4) Å). This is the first example

+

Kr₂F₃in these salts can be attributed to long interionic/

intermolecular Kr‚‚‚F contacts⁵⁰and to the effects of crystal

packing on these highly deformable angles. In contrast, the small

+

(50) The effects of secondary contacts on the geometries of the Kr₂F₃

cations are difficult to assess. The number of Kr‚‚‚F contacts to the

Kr₂F₃⁺cations range from 6 to 8 per krypton atom with Kr‚‚‚F contact

distances ranging from 2.902 to 3.489 Å. There are also 6-8 long

Kr‚‚‚F contacts for the adducted KrF₂molecules, and they vary from

3.104 to 3.473 Å. Of the 58 (Kr₂F₃⁺) and 14 (KrF₂) Kr‚‚‚F contacts

observed in the structures of [Kr₂F₃][SbF₆]‚KrF₂, [Kr₂F₃]₂[SbF₆]₂‚KrF₂,

and [Kr₂F₃][AsF₆]‚[KrF][AsF₆], 50 (Kr₂F₃⁺) and 12 (KrF₂) contacts

have F_t-Kr‚‚‚F angles falling in the ranges 52-80° and 100-122°,

while the remaining long contact angles fall between 80 and 100°.

The angular distribution of long contacts is generally consistent with

avoidance of the three valence lone pair domains of krypton.

+

of an asymmetric Ng₂F₃(Ng ) Kr, Xe) cation that has been

documented crystallographically, although the possibility of an

asymmetric Kr₂F₃⁺cation in [Kr₂F₃][SbF₆] had been previously

3008 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

This trend is consistent with the valence bond (Structures I-III)-

and 3c-4e descriptions of KrF₂, which predict formal Kr-F

bond orders of one-half for KrF₂and one for the free KrF⁺

+

cation. The Kr-F_tbond length in Kr₂F₃is expected to be

intermediate with respect to those of KrF₂and KrF⁺and

consistent with the dominant roles KrF⁺and KrF₂play in the

+

resonance description of Kr₂F₃(Structures IV-VI) and with

a description that invokes a significant contribution from the

fully ionic resonance Structure V. Moreover, the Kr- -F_bbond

(2.027(5)-2.065(4) Å), though significantly longer than that

of KrF₂, has substantial covalent character and is ca. 1.4 Å

shorter than the sum of the krypton and fluorine van der Waals

radii. The covalency of the Kr- -F_bbond is also reflected in the

Kr- -F_b- -Kr bridge angles, which are significantly bent in all

+

three crystal structures containing the Kr₂F₃cation. The bent

angles are consistent with AX₂E₂VSEPR arrangements at their

respective fluorine bridge atoms but are more open than the

ideal tetrahedral angle. The upper limit of the Kr- -F_b- -Kr bond

angle (range, 126.0(2)-142.4(3)°) is in good agreement with

the Xe- -F_b- -Xe bond angles observed in the monoclinic (148.6-

(4)°)^37,38and trigonal (139.8(8)°)³⁸phases of [Xe₂F₃][AsF₆] but

is considerably smaller than the angle observed in [Xe₂F₃][SbF₆]

(160.3(3)°).³⁸The X-ray crystal structures and energy-minimized

structures derived from electron structure calculations for the

+

38

+

Xe₂F₃cation and the Kr₂F₃cation in this work show that

they are W-shaped (see Computational Results), with the F_t-

Xe- -F_bangles distorted from linearity by similar amounts

(experimental, 2.4(3)° [As] and 1.4(4)° [Sb]; theoretical, 2.3°)

when compared with those of Kr₂F₃(experimental, 3.2(1)-

2.1(2)°; theoretical, 2.6°).

Figure 3. (a) Structure of [Kr₂F₃][SbF₆]‚KrF₂and (b) view of the

[Kr₂F₃][SbF₆]‚KrF₂unit cell along the a-axis; thermal ellipsoids are

shown at the 50% probability level.

+

F_t-Kr- -F_bangle distortions that give rise to the W-shaped Kr₂F₃

cations in these salts are not attributed to secondary Kr‚‚‚F

contacts and crystal packing because they persist in the gas-

phase energy-minimized geometry (see Computational Results).

Although the crystal structures of the related XeF⁺salts are

not as precise as those determined for the KrF⁺salts described

in the present study, the trends in geometrical parameters are

clearly analogous. The Xe-F_tbond lengths in the XeF⁺salts

of AsF₆^-, Sb₂F₁₁^-, and RuF₆^-are 1.873(6), 1.82(3), and 1.87-

(2) Å, and the Xe- -F_bbond lengths are 2.212(5), 2.34(3), and

2.18(2) Å, respectively.^33-35At the 3σ limit, the Xe-F_tand

Xe- -F_bbond lengths, like their counterparts in [KrF][MF₆],

exhibit no crystallographically significant dependence on the

counteranion. The F_t-Ng- -F_bangles in both series of salts are

very close to linear. Although the distortions from linearity are

significant in the krypton series, the uncertainties are too high

in the xenon structures to differentiate. The Ng- -F_b- -M angles

in both series are significantly bent and are consistent with

AXYE₂VSEPR arrangements at their respective fluorine bridge

atoms, which, because of the high ionic characters of these

bonds, are significantly more open than the ideal tetrahedral

angle. The Xe- -F_b- -M angles in the XeF⁺salts, 137.2(5)

Unlike the KrF₂molecules in R-KrF₂and â-KrF₂, which are

linear by symmetry, the KrF₂molecules in [Kr₂F₃][SbF₆]‚KrF₂

and [Kr₂F₃]₂[SbF₆]₂‚KrF₂are slightly bent (178.0(2) and 179.1-

(2)°) with Kr-F bond lengths of 1.868(4), 1.888(4) Å and 1.881-

(4), 1.887(4) Å, respectively. The small bond length differences

and angle distortions are likely attributable to long Kr‚‚‚F

contacts⁵⁰and to crystal packing, and do not persist in the gas-

phase energy-minimized geometry (see Computational Results).

The X-ray structure determinations of [Kr₂F₃]₂[SbF₆]₂‚KrF₂and

[Kr₂F₃][SbF₆]‚KrF₂confirm earlier Raman spectroscopic studies

of the KrF₂/SbF₅and KrF₂/AsF₅systems in which an undeter-

mined excess of KrF₂was shown to associate with [Kr₂F₃][SbF₆]

and [Kr₂F₃][AsF₆].^13,15,16The symmetric stretch of the associated

KrF₂was shifted to higher frequency of that of free KrF₂by

∼10 cm^-1in these studies.

+

Structural Comparisons Among NgF₂, NgF⁺, and Ng₂F₃

(RuF₆)

^{- 34}and 134.8(2)° (AsF₆^-),³⁵are in good agreement with

(Ng ) Kr, Xe). Experimental trends in bond lengths and bond

those of [KrF][MF₆] [133.7(1) (As), 139.2(2) (Sb), 138.(3)° (Bi)]

and contrast with those of [KrF][AsF₆] (124.6(3)°) in [Kr₂F₃]-

[AsF₆]‚[KrF][AsF₆] and [XeF][Sb₂F₁₁] (149(2)°).³³The differ-

ences likely arise from differences in the crystal packings and

the deformabilities of the Ng- -F_b- -M angles (see Computational

Results).

+

angles among [NgF][MF₆] ion pairs, NgF₂, and Ng₂F₃salts

are analogous for krypton and xenon. The Kr-F_tbond exhibits

a considerable variation in length among the solid-state krypton

fluoride structures that are now known. The Kr-F_tbonds in

the KrF⁺salts (1.765(3)-1.783(6) Å) and in the Kr₂F₃salts

+

(1.780(7)-1.805(5) Å) show no crystallographically significant

differences and are longest in KrF₂(1.868(4)-1.889(10) Å).

Synthesis and Characterization by Raman Spectroscopy

of [Kr₂F₃][PF₆]‚nKrF₂. Neither [KrF][PF₆] and [Kr₂F₃][PF₆]

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3009

Figure 4. (a) Structure of [Kr₂F₃]₂[SbF₆]₂‚KrF₂and (b) view of the [Kr₂F₃]₂[SbF₆]₂‚KrF₂unit cell along the a-axis; thermal ellipsoids are shown

at the 50% probablility level.

nor their xenon(II) analogues have been reported previously.

and were verified with the aid of theoretical calculations (see

Computational Results). The PF₆^-anion spectrum was identified

by the A_1g(748 cm^-1), E_g(572 and 581 cm^-1), and T_2g(464,

469, and 475 cm^-1) modes, which are consistent with an

octahedral anion for which site symmetry lowering apparently

results in removal of the degeneracies of the E_gand T_2gmodes.⁵¹

The intense bands at 555 and 605 cm^-1are assigned under C_2V

symmetry to the asymmetric and symmetric Kr-F_tstretches,

+

Attempts, in the present work, to synthesize KrF⁺and Kr₂F₃

salts of the PF₆^-anion by reaction of KrF₂with excess PF₅in

anhydrous HF solvent at -78 °C failed. Reaction of KrF₂with

liquid PF₅at -78 °C, however, yielded [Kr₂F₃][PF₆]‚nKrF₂,

where n is unspecified. The complete conversion to [Kr₂F₃]-

[PF₆]‚nKrF₂required nearly 2 months and was deemed complete

upon the disappearance of the intense ν₁(Σ⁺_g) vibrational mode

of free KrF₂at 465.5 cm^-1(R-phase) or 468.6 and 469.5 cm^-1

(â-phase) in the Raman spectrum, during which time modes

consistent with Kr₂F₃⁺and PF₆^-became more intense. Further

addition of fresh aliquots of PF₅to [Kr₂F₃][PF₆]‚nKrF₂did not

result in [KrF][PF₆] or any further changes in the Raman

spectrum over an additional 2 month period. The salt was

isolated at -78 °C as a white solid under 1000 Torr of PF₅(g).

The [Kr₂F₃][PF₆]‚nKrF₂adduct rapidly dissociated to KrF₂and

PF₅under reduced PF₅pressures at -78 °C.

+

ν₆(B₁) and ν₁(A₁), respectively, of the Kr₂F₃cation. It is

noteworthy that, like [Kr₂F₃][AsF₆]‚nKrF₂and [Kr₂F₃][SbF₆]‚

nKrF₂, the 605 cm^-1band is not factor-group split but is split

in [Kr₂F₃][AsF₆] and [Kr₂F₃][SbF₆]. The weak band at 358 cm^-1

is assigned to ν₂(A₁), the symmetric Kr- -F_b- -Kr stretch, and

the coupled in-plane and out-of-plane bends associated with the

F_t-Kr- -F_bgroups are assigned to the medium intensity bands

ν₉(B₂) 191, ν₃(A₁) 185, and ν₅(A₂) 176 cm^-1. The aforemen-

tioned show negligible dependence on the counteranion, which

-

is also true of the AsF₆and the SbF₆salts,^13,16and is

+

consistent with a Kr₂F₃cation that interacts weakly with the

PF₆anion. The Raman spectrum (Figure 6) also revealed a

The Raman spectrum of [Kr₂F₃][PF₆]‚nKrF₂(Figure 6) was

acquired under 1000 Torr of PF₅at -80 °C after 7 weeks of

contact between KrF₂and PF₅at -78 °C and closely resembles

the spectra of the [Kr₂F₃][MF₆]‚nKrF₂(M ) As, Sb) salts (Table

5). The assignments of the [Kr₂F₃][PF₆]‚nKrF₂spectrum were

consequently based on those of the AsF₆^-and SbF₆^-analogues

-

trace impurity in the sample, which exhibited weak vibrational

frequencies at 1859 and 1863 cm^-1. The bands are assigned to

+

the O₂cation and are in good agreement with the O₂

(51) Heyns, A. M. Spectrochim. Acta 1977, 33A, 315.

3010 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

Figure 5. (a) Structure of [Kr₂F₃][AsF₆]‚[KrF][AsF₆] and (b) view of the [Kr₂F₃][AsF₆]‚[KrF][AsF₆] unit cell along the a-axis; thermal ellipsoids

are shown at the 50% probability level.

frequencies observed for [O₂][MF₆] (M ) As, Sb, Bi, Ru, Rh,

Pd, Pt, Au).⁵²The [O₂][PF₆] contaminant arises from the

oxidation of trace amounts of H₂O and/or O₂by the strongly

oxidizing Kr₂F₃⁺cation.¹⁶The band splitting is likely attribut-

able to a mixture of R- and â-[O₂][PF₆] that apparently coexist

near the phase-transition temperature as noted for [O₂][AsF₆]^53,54

and [O₂][AuF₆].⁵⁵

geometries and electronic properties of Xe₂F₃⁺and the isoelec-

tronic analogue, XeIF₃, defining for the first time the 5c-6e

hypervalent bond. In these studies, the HF method predicted a

linear structure for Xe₂F₃⁺. This contrasts with the bent structure

observed for Xe₂F₃⁺in its crystal structures in which the Xe- -

F_b- -Xe bond angle ranges from 139.8(8) to 160.3(3)°.^37,38In a

more recent study, the local density-functional theory (LDFT)

results for Xe₂F₃⁺are in better agreement with the experiment

than the HF values, predicting a bent Xe- -F_b- -Xe angle

(146.9°).³⁸In the present study, we have used both HF and

LDFT electronic structure calculations to characterize KrF₂,

KrF⁺, Kr₂F₃⁺, and the fluorine-bridged ion pairs [KrF][MF₆]

(M ) P, As, Sb, Bi).

Computational Results

Dixon et al.⁵⁶have previously used ab initio calculations at

the Hartree-Fock (HF) level to obtain the energy-minimized

(52) Griffiths, J. E.; Sunder, W. A.; Falconer, W. E. Spectrochim. Acta

1975, 31A, 1207.

(53) Naulin, C.; Bougon, R. J. J. Chem. Phys. 1976, 64, 4155.

(54) Griffiths, J. E.; Sunder, W. A. J. Chem. Phys. 1982, 77, 1087.

(55) Lehmann, J. F.; Schrobilgen, G. J. Unpublished results.

(56) Dixon, D. A.; Arduengo, A. J., III.; Farnahm, W. B. Inorg. Chem.

1989, 28, 4589.

Geometries. (a) KrF₂. The optimized HF and LDFT

geometries of KrF₂(Table 2) are linear with Kr-F bond lengths

of 1.822 and 1.910 Å, respectively, with the LDFT method

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3011

+

Table 5. Experimental Raman and Calculated Vibrational Frequencies, Assignments, and Mode Descriptions for Kr₂F₃

+

[Kr₂F₃][PF₆]‚

[Kr₂F₃][AsF₆]‚

[Kr₂F₃][SbF₆]‚

assignments for Kr₂F₃

b,c

a,b

[Kr₂F₃][AsF₆]^a,b

[Kr₂F₃][SbF₆]^a,b

nKrF₂

LDFT^d

in C_2Vpoint group^e

610(43), 600(80),

594(100)

570(4), 567(31)

437(5)

347(sh), 336(17)

183(15)

603(100), 594(89)

605(100)

602(100)

599(100)

628(12)

ν₁(A₁) ν(KrF_t+ KrF_t)

555(34)

456(4)

330(18)

186(16)

555(52)

462(11)

358(19)

191(11)

575(23), 553(50)

462

355(19)

557(50)

466(60)

340(14)

200(2)

608(273)

441(212)

313(11)

196(14)

ν₆(B₁) ν(KrF_t- KrF_t)

ν₇(B₁) ν_as(KrF_b)

ν₂(A₁) ν_s(KrF_b)

ν₉(B₂) δ(F_t-Kr- - -F_b)

oop antisym comb

ν₃(A₁) δ_s(F_t-Kr-F_b)

ip sym comb

ν₅(A₂) δ(F_t-Kr- - -F_b)

oop sym comb

ν₈(B₁) δ(F_t-Kr- - -F_b)

ip antisym comb

190(10)

174(13)

158(2)

180(sh)

176(sh)

185(7)

176(5)

184(sh)

177(sh)

188(10)

122(46)

168(1)

159(0)

155(0)

41(0)

ν₄(A₁) δ(Kr- - -F_b- - -Kr)

bend

^aFrom ref 16; anion modes are also given in this reference. The abbreviation, sh, denotes a shoulder. ^bValues in parentheses denote relative

Raman intensities. ^cFrequencies observed for PF₆^-: 748(7) ν₁(A_1g); 581(7) and 572(13), ν₂(A_1g); 475(12), 469(35), and 464(11), ν₅(T_2g). Spectrum

1

recorded on a powder in a /₄-in. FEP sample tube at -80 °C using 514.5 nm excitation. Values in parentheses denote relative Raman intensities.

Additional weak bands were observed at 1859(0.20) and 1863(0.15) cm^-1that are assigned to O₂

parentheses. ^eThe abbreviations oop and ip denote out of plane and in plane, respectively.

+

.

^dInfrared intensities, in km mol^-1, are given in

and consequently, their deviations from the calculated values

are attributed to crystal packing.

The bond length of free KrF⁺was calculated to be 1.780 Å

(LDFT) and 1.717 Å (HF). The LDFT values for the Kr-F_t

bond length do not show as pronounced variations for the

structures of the ion pairs (1.857-1.877 Å) as the HF values

do, which predict a longer bond in [KrF][PF₆] (1.789 Å) than

in [KrF][AsF₆] (1.746 Å), [KrF][SbF₆] (1.739 Å), and [KrF]-

[BiF₆] (1.739 Å). Surprisingly, the experimental Kr-F_tbond

lengths are most accurately approximated by the bond length

of free KrF⁺, which is overestimated by the same magnitude

(∼0.01 Å) as those calculated for KrF₂and the Kr-F_tbond

lengths of Kr₂F₃⁺using LDFT. It should be noted that the LDFT

Kr-F_tbond lengths in the ion pairs are longer than the free

Figure 6. Raman spectrum of powdered [Kr₂F₃][PF₆]‚nKrF₂recorded

at -80 °C using 514.5 nm excitation. Peaks arising from the FEP

sample tube are denoted by asterisks (*), while that resulting from

excess PF₅is denoted by a dagger (†).

predicting longer distances and providing better agreement with

the experimental values.

(b) KrF⁺and [KrF][MF₆]. Two conformers having C_spoint

symmetry are found for the energy-minimized gas-phase

structures of the [KrF][MF₆] ion pairs (Table 3). The theoretical

structures for [KrF][PF₆] and [KrF][AsF₆] have the F_t-Kr- -F_b

moiety in an MF₃plane (Structure VII), whereas for [KrF][SbF₆]

KrF⁺value by ∼0.09 Å and are also predicted to be longer

+

than the Kr-F_tbond lengths of Kr₂F₃

.

LDFT predicts that the KrF₂interaction with the weak fluoride

ion acceptor, PF₅, is better described as a KrF₂‚PF₅adduct than

the ionic complex [KrF][PF₆]. The KrF₂moiety in this species

is asymmetric with a Kr- -F_bbond length of 1.972 Å and a Kr-

F_tbond length of 1.877 Å. The Kr- -F_bbond is 0.06 Å longer,

and the Kr-F_tbond is 0.03 Å shorter than the calculated bond

distance in KrF₂, while Kr-F_tin KrF₂‚PF₅is 0.10 Å longer

than the bond in free KrF⁺. The P- -F_bbond distance of 2.176

Å is lengthened by 36% when compared with the average bond

length of the equatorial P-F bonds (1.595(9) Å). The M- -F_b

bridge distances for the heavier pnicogens in [KrF][MF₆] are

2.096 (As), 2.185 (Sb), and 2.266 (Bi) Å, and the Kr- -F_bbridge

bond distances are 1.998 (As), 2.017 (Sb), and 2.012 (Bi) Å

with shorter terminal Kr-F_tbond distances of 1.867 (As), 1.857

(Sb), and 1.859 (Bi) Å. The experimental structures likewise

fail to show a consistent trend among Kr- -F_bdistances [2.131-

(2) (As), 2.140(3) (Sb), and 2.089(6) (Bi) Å] and Kr-F_t[1.765-

(2) (As), 1.765(3) (Sb), and 1.774(6) (Bi) Å] bond lengths. The

and [KrF][BiF₆], the F_t-Kr- -F_bmoiety is rotated by 45° so that

it is staggered and lying in the mirror plane that bisects the

angle subtended by two equatorial fluorine atoms of the axially

distorted MF₆^-anion (Structure VIII). The staggered structure

is more stable than the eclipsed one by 0.7 kcal mol^-1for [KrF]-

[SbF₆] and by 0.8 kcal mol^-1for [KrF][BiF₆]. In contrast, this

angle ranges from 19.7 to 28.1° in the experimental structures,

3012 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

calculated M- -F_bdistances are 21 (As), 14 (Sb), and 15% (Bi)

longer than the average of the equatorial M-F bond lengths:

1.737(10) (As), 1.916(10) (Sb), and 1.967(12) (Bi) Å. The

comparison is particularly noteworthy for P and As, which have

similar averaged equatorial M-F bond lengths. The degree of

fluoride ion transfer from KrF₂to the strong fluoride ion

acceptor, AsF₅, is considerably greater than for PF₅. The M- -

F_binteractions and degrees of F^-transfer are greatest for SbF₅

and BiF₅, where the M- -F_bbond lengths are only ∼15% longer

than the average equatorial M-F bond length. This trend is

consistent with the fluoride ion affinities of the parent MF₅

molecules, which have been calculated to be PF₅(94.9 kcal

mol^-1) < AsF₅(105.9 kcal mol^-1) < SbF₅(120.3 kcal mol^-1).⁵⁷

Comparison of the calculated structures with the experimental

structures shows that the degree of fluoride ion transfer from

MF₆^-to KrF⁺in the crystal is substantially less than is predicted

for the gas-phase species. The experimental M- -F_bbond lengths

were determined to be 1.845(2), 1.963(3), and 2.105(6) Å for

As, Sb, and Bi, respectively, and are only 8, 6, and 8% longer

than the average of their respective experimental equatorial

M-F bonds.

and lie in the Kr- -F_b- -Kr plane to give a W-shaped structure.

The persistence of these bond angles in the gas phase indicates

that they are intrinsic properties of the cation, although packing

effects and long contacts in the solid state are undoubtedly

responsible for the bond angle variations observed among the

different salts.

The nonlinear F_t-Kr- -F_bbond angles result from a number

of factors. The overall molecule is bent with a Kr- -F_b- -Kr bond

angle of 135° and an asymmetric environment about the krypton

with two inequivalent fluorine atoms. Thus, there is no

symmetry constraint to keep the F_t-Kr- -F_bbond angle linear.

The bond angle deviates by less than 3° from 180°, leading to

an increase in the distance between F_tatoms. We note that the

molecule is quite floppy with F_t-Kr- -F_bbending frequencies

less than 170 cm^-1. The deviation from linearity likely arises

because the bonding about krypton is no longer as symmetric

as it is in KrF₂. Thus, the 3c-4e hypervalent bond at krypton

can build in other orbital character and deviate from linearity.

The bonding in Kr₂F₃has a strong component of two KrF⁺

+

cations bonding to the central F^-ion. With no orbital symmetry

constraint, the bond angles at the Kr can adjust to maximize

the interactions with F_band minimize any repulsive interactions.

Vibrational Frequencies. The vibrational frequencies of

KrF₂, Kr₂F₃⁺, KrF⁺, and the [KrF][MF₆] ion pairs have been

determined using HF and LDFT calculations. Comparison of

the two methods reveals that the vibrational frequencies

predicted by the LDFT method are more accurate than those

predicted by the HF method. HF theory was found to overes-

timate the vibrational frequencies by 12-143 cm^-1, in accord

with the shorter bond lengths predicted at this level of theory.

Moreover, the geometry of Kr₂F₃⁺calculated at the HF level is

linear and not bent as arrived at by LDFT and observed

experimentally. Despite the significant difference in the LDFT

and HF bond lengths and bond angles, the frequency orderings

are essentially identical using the two levels of theory. Only

the vibrational frequencies predicted by LDFT theory are

compared in the ensuing discussion.

The F_t-Kr- -F_bangles in the [KrF][MF₆] ion pairs are

predicted to be slightly distorted [178.3 (P), 177.7 (As), 177.4

(Sb), and 177.2 (Bi)°] from linearity, and similar angle distor-

tions are observed in the experimental structures [176.8(1) (As),

177.9(2) (Sb), and 177.0(4) (Bi)°]. The Kr- -F_b- -M angles [120.0

(As), 114.1 (Sb), and 113.8 (Bi)°] are predicted to be more

closed than in the experimental structures [133.7(1) (As), 139.2-

(2) (Sb), and 138.3(3) (Bi)°], where crystal packing apparently

requires more open angles. The consistency of the experimental

angles is likely a consequence of their isomorphous packing

arrangements. The explanation is supported by the observation

that the Kr- -F_b- -As angle (124.6(3)°) in the [KrF][AsF₆] unit

of the nonisomorphous double salt, [Kr₂F₃][AsF₆]‚[KrF][AsF₆],

is more closed than in â-[KrF][AsF₆] and is in better agreement

with the theoretical value. These angles are expected to be highly

deformable and susceptible to crystal packing effects, on the

basis of the low calculated gas-phase frequencies of their

(a) KrF₂. LDFT predicts the vibrational modes of KrF₂to

occur at ν₁(Σ⁺_u) 623, ν₂(Σ⁺_g) 504, and ν₃(Π_u) 228 cm^-1, which

are in reasonable agreement with those found experimentally

(596/580, 449, and 233 cm^-1).⁵⁸The Kr-F stretch of free KrF⁺

is predicted to be at 686 cm^-1and is significantly higher than

the symmetric stretching frequency of KrF₂. The difference is

consistent with the 2c-2e and 3c-2e descriptions of KrF⁺and

KrF₂, which lead to formal bond orders of 1 and ¹/₂, respectively.

The experimental Kr-F stretching frequencies for the KrF⁺salts

we have considered in the present work range from 607 to 619

cm^-1. The high value of the stretching frequency of the free

KrF⁺cation, relative to that of coordinated KrF⁺in its salts, is

consistent with a significant fluorine bridge interaction between

the cation and anion, which leads to longer and more ionic Kr-

F_tbonds in the ion pairs.

bending modes [73 (As), 80 (Sb), and 74 (Bi) cm^-1].

+ 38

(c) Kr₂F₃⁺. As in a previous study of Xe₂F₃

,

the LDFT

+

method predicts a bent structure for Kr₂F₃in agreement with

the experiment, whereas the HF method predicts a linear

structure (Table 4). As expected from the results for KrF₂, the

LDFT method predicts the Kr-F_tbond lengths to be too long

by ∼0.03 Å. Similarly, the Kr- -F_bbond length is predicted to

be too long by 0.02-0.05 Å, depending on the structure. The

linear structure predicted by the HF method has a Kr-F_tbond

distance that is too short (1.730 Å) and a Kr- -F_bbond distance

that is too long (2.082 Å).

As previously noted for KrF₂, the predicted bond lengths

bracket the Kr-F_tbond length with a slight overestimation

(∼0.02 Å) by LDFT and a larger underestimation (∼0.07 Å)

by HF theory. Both LDFT and HF theory overestimate the Kr- -

F_bbond length, arriving at similar values of 2.081 and 2.082

Å, respectively. In a previously published study, LDFT was

shown to predict F_t-Xe- -F_b(178.3°) and Xe- -F_b- -Xe (146.9°)

(b) Kr₂F₃⁺. The calculated LDFT vibrational frequencies of

+

the Kr-F_tstretches of Kr₂F₃(Table 5) are also significantly

lower than the Kr-F stretching frequency of free KrF⁺and are

consistent with a significant bonding interaction between the

KrF⁺cations and the bridging F^-. The calculated values are in

excellent agreement with the experimental frequencies, with

calculated values for the Kr-F_tstretching modes at ν₆(B₁), 608

and ν₁(A₁), 628 cm^-1and for the Kr- -F_bstretching modes at

ν₂(A₁), 313 and ν₇(B₁), 441 cm^-1. The previous experimental

assignments for ν₁(A₁) and ν₆(B₁) are, however, reversed with

bond angles in Xe₂F₃with reasonable accuracy.³⁸Likewise,

+

LDFT predicts F_t-Kr- -F_band Kr- -F_b- -Kr bond angles of 177.4

and 135.2°, respectively, which lie within their respective

experimental bond angle ranges of 175.1(2)-178.6(3) and

126.0(2)-142.4(3)°. Moreover, in both the experimental and

calculated structures, the F_tatoms are bent away from each other

(57) Christe, K. O.; Dixon, D. A.; McLemore, D. K.; Wilson, W. W.;

(58) Claassen, H. H.; Goodman, G. L.; Malm, J. G.; Schreiner, F. J. Chem.

Phys. 1965, 42, 1229.

Sheehy, J. A.; Boatz, J. A. J. Fluorine Chem. 2000, 101, 151.

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3013

Table 6. Calculated Vibrational Frequencies, Assignments, and

Mode Descriptions for [KrF][PF₆]

respect to the theory-based assignments and have been reas-

signed accordingly in Table 5. The F_t-Kr- -F_bin-plane and out-

of-plane bending modes are in good agreement with the

experimental values. The in-plane bending frequency corre-

sponding to the deformation of the Kr- -F_b- -Kr angle is very

low, 41 cm^-1. Thus, the various interactions within the crystal

could easily distort this angle because the potential energy

surface for bending is so flat. Similar deformabilities have been

frequencies (cm^-1

LDFT^aHF^a

)

assignments (C_s)^b

951(358) 1085(437) A′′ ν_as(PF_2e)

937(305) 1071(413) A′ ν(PF′_e)

917(375) 1052(382) A′ ν(PF_a)

735(8)

616(71)

859(8)

694(5)

A′ ν_s(PF₅)

A′ ν_s(PF_2e) - ν_s(PF′_e) - ν_s(PF′′_e) + ν(KrF_t)

noted for the Xe- -F_b- -Xe angles in several Xe₂F₃salts^37,38

+

613(100) 676(250) A′ ν_s(PF_2e) - ν_s(PF′_e) - ν_s(PF′′_e) - ν(KrF_t)

when compared with their theoretical gas-phase values.

(c) [KrF][MF₆]. A difficulty arises when assigning the

vibrational spectra of the ion pairs. In the idealized case of

515(32)

511(38)

499(1)

598(109) A′ δ(PF_2e) + δ(F_aPF′_e- F_aPF′′_e)

592(57) A′′ δ(F′_ePF′′_e) + δ(F_aPF_e- F_aPF_e)

587(5)

A′′ δ(F"_ePF_e) + δ(F′_ePF_e)

-

497(21)

580(27) A′ δ(PF_2e) + δ(PF′_eF′′_e) oop + ν_s(KrF₂)

complete fluoride ion transfer, an octahedral MF₆anion (O_h

436(113) 505(283) A′ ν(KrF′′_e) + ν(KrF_b) + δ(PF_2eF′_eF′′_e) oop

358(0)

341(20)

point symmetry) results, and only the Raman-active ν₁(A_1g),

ν₂(E_g), and ν₅(T_2g) modes and the infrared-active ν₃(T_1u) and

ν₄(T_1u) modes are observed in addition to ν(Kr-F) of the cation.

Because complete fluoride ion transfer is never achieved in the

solid state, except when the cation is coordinately saturated as

402(6)

387(0)

A′′ δ(F_ePF_a) - δ(F_aPF_e) + δ(F_ePF_b) - δ(F_ePF_b)

A′ δ(F_aPF′′_e) ip - δ(F_aPF′_e) ip

278(117) 312(83) A′ ν(KrF_b) + δ(F_tKrF_b) ip

247(6)

190(0)

164(3)

155(6)

127(17)

81(0)

304(20) A′′ δ(F_tKrF_b) oop

149(0)

145(5)

112(5)

A′ δ(F′_ePF′′_e) ip - δ(F_ePF_e) along F_a-P-F_b

A′′ δ(F_tKrF′′_e) oop + torsion about PF′′_e

A′ δ(F′_ePF′′_e) - δ(F_tKrF′′_e) ip

+

in Kr₂F₃salts, the anion symmetry is primarily distorted by

the elongation of the bond involved in fluorine bridging to the

cation, namely, M- -F_b. The secondary fluorine bridge interaction

results in lowering of the anion symmetry and the observation

of additional bands in the vibrational spectra. The symmetry

lowering is oftentimes approximated as C_4Vsymmetry,^14-17,59,60

and the vibrational spectra are assigned under this or a lower

point symmetry such as C_ssymmetry^59,60to allow for the bent

nature of the Ng- -F_b- -M moiety (where Ng is the central Kr

or Xe atom of the cation). Along with the tentative assignments

and approximate mode descriptions for the fluorine-bridged

anion, attempts have been previously made to assign the

vibrational modes associated with the Ng- -F_b- -M bridge.^14-17,59,60

We now report the mode descriptions and the frequency

assignments derived from LDFT and HF calculations for the

fluorine-bridged F_t-Kr- -F_b- -MF₅ion pairs. The assignments and

their descriptions (Tables 6-9) are for eclipsed (M ) P, As;

Structure VII) and staggered (M ) Sb, Bi; Structure VIII) gas-

phase conformations having C_spoint symmetries. Although the

conformations of the ion pairs are skewed to give C₁point

symmetry in their crystal structures and not all of the predicted

frequencies have been observed, the approach provides more

complete assignments and more precise descriptions of the

vibrational modes in these strongly coupled systems than have

been available previously.¹⁶Assignments and descriptions of

the vibrational modes of the MF₅moiety are provided without

further comment.

75(10) A′ ν(P-F_b)

52(3)

10i(0)

A′ δ(PF_bKr) + ν(KrF′′_e)

A′′ torsion of KrF₂group about the P-F_bbond

+ rock of PF₅group toward KrF₂

23(0)

^aInfrared intensities, in km mol^-1, are given in parentheses. ^bThe

atom labeling scheme is given in Structure VII; oop and ip denote out

of plane and in plane, respectively.

Table 7. Experimental Raman Frequencies and Calculated

Vibrational Frequencies, Assignments, and Mode Descriptions for

â-[KrF][AsF₆]

frequencies (cm^-1

)

LDFT^a

HF^a

exp^b

assignments (C_s)^c

A′′ ν_as(AsF_2e)

A′ ν(AsF′_e) - ν(AsF′′_e) - ν(AsF_a)

A′ ν(AsF′_e) - ν(AsF′′_e) + ν(AsF_a)

A′ ν_s(AsF₅) + small ν(AsF_a)

745(152) 824(199)

737(129) 819(192)

730(123) 813(114) 716(16)

652(16) 745(57) 676(13)

611(179) 735(113) 615(100), A′ ν(KrF_t) - ν(KrF_b)

619(72)

601(0) 650(4)

454(23) 498(128) 464(3)

411(10),

595(5)

A′ ν(AsF′′_e+ AsF′_e) - ν(AsF_2e)

A′ ν(KrF_b) - ν(AsF_b) + some ν(KrF_t)

unassigned AsF₆^-bands^d

419 sh

361(16) 447(63) 384(4)

356(14) 434(52) 366(9)

A′′ δ(F_eAsF′′_e) - δ(F_eAsF′′_e)

A′ δ(F_bAsF′′_e) + δ(F_eAsF_e) along

F′_e-As-F′′_e

A′′ δ(F′_eAsF_e) - δ(F′_eAsF_e)

{

A′ δ(F_eAsF_e) + δ(F′′_eAsF_b) along

F′_e-As-F′′_e

A′ ν(AsF_b) + δ(AsF_2eF′_eF′′_e) oop

A′′ δ(F_bAsF_e) + δ(F_aAsF_e)

A′ δ(F′′_eAsF_a) + δ(F′_eAsF_b) + ν(F_bKr)

A′′ δ(F_tKrF_b) oop + δ(F_bKrF_a)

A′ δ(F_eAsF_e) - δ(F′′_eAsF_e)

A′ δ(F_tKrF′′_e) ip + ν(AsF_b)

A′′ δ(F_tKrF_b) oop + (AsF₅) rock oop

A′ δ(F′′_eAsF_b) - δ(F′_eAsF_b)

A′ δ(KrF_bAs)

340(25) 425(35)

335(24) 406(13)

}

338(16)

273(1)

289(199) 365(0)

280(0) 315(157)

249(7) 299(376)

225(5) 282(5)

171(1) 222(1)

155(6) 197(3)

149(1) 185(41)

137(1) 168(9)

The calculated Kr-F_tstretches of the gas-phase [KrF][MF₆]

ion pairs occur at 611 (As), 616 (Sb), and 616 (Bi) cm^-1and

are in excellent agreement with the experimental values, which

are also insensitive to the degree of F^-ion transfer. The lower

frequency Kr- -F_bstretches, which are predicted at 454 (As),

357 (Sb), and 436 (Bi) cm^-1, are in good agreement with the

experiment. Direct comparison of the Kr- -F_bstretching mode

is not possible because of significant coupling to Kr-F_t(As)

and to MF₅modes (Sb). Only in [KrF][BiF₆] is the Kr- -F_b

stretch not significantly coupled. The Kr-F_tand Kr- -F_b

173(10)

162(11)

73(0)

4(0)

70(0)

21i(0)

A′′ torsion of KrF₂group about the

As-F_bbond + rock of AsF₅group

toward KrF₂

^aInfrared intensities, in km mol^-1, are given in parentheses.

^bExperimental values are taken from ref 16. The abbreviation, sh,

denotes a shoulder. Values in parentheses denote relative Raman

intensities. ^cThe atom labeling scheme is given in Structure VII; oop

and ip denote out of plane and in plane, respectively. ^dSimilar bands

-

stretches of the PF₆compound, for which there are no

experimental values, are both strongly coupled to anion modes

and cannot be compared with other members of the series.

Mayer Bond Orders and Valencies and Mulliken Charges.

One way to gain insight into the types of interactions involved

is to use Mayer valency and bond order indices.^61-64The Mayer

-

have been observed in other AsF₆salts but are not assigned.

bond orders, Mayer valencies, and atomic charges for KrF₂,

KrF⁺, Kr₂F₃⁺, and the [KrF][MF₆] ion pairs have been estimated

using the electronic structures derived from LDFT calculations

(Table 10). The Kr-F_tbond orders were found to have little

(59) Sladky, F. O.; Bulliner, P. A.; Bartlett, N. J. Chem. Soc. A 1969, 2179.

(60) Gillespie, R. J.; Landa, B.; Schrobilgen, G. J. Inorg. Chem. 1976, 15,

1256.

3014 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

Table 9. Experimental Raman Frequencies and Calculated

Vibrational Frequencies, Assignments, and Mode Descriptions for

[KrF][BiF₆]

Table 8. Experimental Raman Frequencies and Calculated

Vibrational Frequencies, Assignments, and Mode Descriptions for

[KrF][SbF₆]

frequencies (cm^-1

LDFT^aHF^aexp^b

)

frequencies (cm^-1

LDFT^aHF^a

)

assignments^c(C_s)

exp^b

assignments (C_s)^c

668(94) 748(72) 682(3)

A′ ν_as(SbF_e- SbF′_e) + ν_as(SbF_a) +

ν_as(SbF′_e- SbF_e)

616(149) 737(106) 604(11), A′ ν(KrF_t) - small ν(KrF_b)

610(100)

666(94) 727(134) 664(45)

661(72) 725(139) 642(10)

A′′ ν(SbF′_e- SbF_e) + ν(SbF_e- SbF′_e)

A′ ν(SbF_a) - ν(SbF′_e+ SbF′_e)

578(68) 691(83)

592(35), A′ ν(BiF_a)

600(11)

567(60) 690(119)

566(64) 689(93)

537(6) 668(48)

527(8) 614(8)

436(75) 446(368)

342(43) 374(31)

234(22) 293(44)

230(30) 258(0)

A′′ ν_as(BiF_2e) - ν(BiF′_e)

616(170) 719(113) 615(100), A′ ν(KrF_t) + ν(SbF_2e+ SbF′_2e)

580(40)

}

619(74)

A′′ ν_as(BiF_2e) - ν(BiF′_e)

A′ ν_s(BiF_2e+ BiF′_2e)

A′′ ν_as(BiF_2e) + ν_as(BiF′_2e)

A′ ν(KrF_b)

600(5) 677(38)

A′ ν(SbF_2e+ SbF′_2e) + small ν(SbF_a) -

small ν(KrF_t)

A′ ν(SbF_e+ SbF′_e) - ν(SbF_e+ SbF′_e)

A′ ν(KrF_b) + ν(KrF_t)

547(3)

541(9)

581(4) 613(4)

586(5)

439(64) 439(239) 462(8),

316(6)

244(5)

233(3),

225(1)

A′ ν(BiF_b)

A′′ δ(F_bKrF_t) oop - δ(F_bKrF_a)

A′ δ(BiF′_2e)

480(4)

357(43) 413(31) 338(4)

253(47) 331(82) 288(9)

246(44) 327(12) 266(6)

A′ ν(SbF_b)

A′ δ(F′_eSbF′_e)

A′′ δ(F_bSbF_a) oop δ(F_tKrF_b)

A′ δ(F_eSbF_e)

A′ δ(F_aSbF_b) ip + δ(F_eSbF_e) - δ(F′_eSbF′_e)

A′′ δ(F_tKrF_b) oop - δ(F_aSbF_b) oop

A′′ δ(F_tKrF_b) - δ(F_bSbF_a)

A′′ δ(F_aSbF_e) - δ(F_eSbF′_e) + δ(F_tKrF_b)

oop

213(65) 237(203) 209(5)

A′ δ(BiF_2e)

236(60) 311(78)

231(41) 300(19)

226(5) 294(268) 214(2)

179(4) 236(35) 169(5)

205(33) 236(67)

201(35) 234(52)

203(3)

193(4),

184(5)

A′′ δ(F′_eBiF_a) - δ(F′_eBiF_e) - δ(F′_eBiF_e)

A′ δ(F_eBiF′_e) + δ(F_aBiF_b) ip

255(6)

}

{

166(1) 199(1)

163(8) 193(6)

159(1) 184(15)

A′′ δ(F_eBiF_a) + δ(F_tKrF_b) oop

A′′ δ(F_eBiF_a) - δ(F_eBiF_e)

A′ δ(F_tKrF_b) ip + ν(F_bBi) + small

δ(F_bBiF_a)

176(2) 227(2)

162(7)

{

}

173(7)

165(1) 194(2)

139(0) 173(1)

145(3)

A′ δ(F_tKrF_b) ip

A′′ δ(F_aSbF_b) oop + δ(F_eSbF′_e) +

127(0) 134(0)

100(1) 127(2)

84(0) 107(3)

A′′ δ(F_bBiF_a) oop

δ(F_eSbF′_e)

117(4)

87(7)

81(10)

A′ δ(F_bBiF_a) ip + δ(KrF_bBi)

A′′ δ(BiF_bKr) oop

109(2) 152(0)

91(0) 141(11)

A′ δ(F_bSbF_a) ip - δ(SbF_bKr)

A′′ δ(F′_eSbF_e) oop - δ(F′_eSbF_e)

A′ δ(SbF_bKr) + δ(F_bSbF_a)

A′′ KrF_ttorsion about SbF_b+ (SbF₅) rock

74(0)

43(0)

51(1)

6i(0)

A′ δ(BiF_bKr) ip

80(0)

54(0)

53(1)

22(0)

A′′ KrF_ttorsion about Bi-F_b+ (BiF₅)

rock

^aInfrared intensities, in km mol^-1, are given in parentheses.

^bExperimental values are taken from ref 16. Values in parentheses

denote relative Raman intensities. ^cThe atom labeling scheme is given

in Structure VIII; oop and ip denote out of plane and in plane,

respectively.

^aInfrared intensities, in km mol^-1, are given in parentheses.

^bExperimental values are taken from ref 17. Values in parentheses

denote relative Raman intensities. ^cThe atom labeling scheme is given

in Structure VIII; oop and ip denote out of plane and in plane,

respectively.

dependence on the hexafluoropnicogen counteranion despite the

strong fluorine bridge between the two ions and are in agreement

with the constancy of the Kr-F_tbond lengths observed among

the crystal structures of [KrF][MF₆] (M ) As, Sb, Bi) (Table

3) and their calculated values (Table 3).

of charge has shifted from the bridge fluorine onto the Kr-F_t

+

group. The charge on F_t(-0.37) of Kr₂F₃is very similar to

those on the fluorines of KrF₂(-0.36) and that of F_b(-0.16)

is intermediate with respect to that of KrF⁺(0.00) and KrF₂.

The charge distributions indicate that the bridge fluorine of

The atomic charges of KrF₂were calculated to be 0.72 on

krypton and -0.36 on each of the fluorine centers. For KrF⁺,

the full positive charge resides on krypton with zero charge on

the fluorine atom. The Mayer bond order for uncoordinated

KrF⁺is 1.09, and the valence at Kr is 1.09 as compared with

the Kr-F bond order of KrF₂, which is reduced to 0.67 with a

Kr valence of 1.35. There is considerable residual bond order

between the terminal fluorine atoms in KrF₂. This arises because

of the 3c-4e bond that can be used to describe the bonding in

KrF₂. The 3c-4e bond leads to build up of density on the

terminal atoms, and these atoms are linked through the 3c-4e

bond, which is manifested in the residual bond order of 0.22.

The bond orders are similar for both Ng₂F₃⁺cations; however,

the charges and valencies (Xe₂F₃⁺values are taken from ref 38

and are given in square brackets) show some noteworthy

+

Kr₂F₃is, next to the fluorine of KrF⁺, the most electrophilic

fluorine in the series under discussion. There is again a

+

significant bond order for F_t- -F_b(0.14) in the Kr₂F₃cation,

which is reduced with respect to that of KrF₂(0.22) but

+

significantly greater than that in Xe₂F₃(0.08).⁶⁵The 5c-6e

bond that can be used to qualitatively describe the bonding in

Kr₂F₃⁺shows that there is an interaction between the F_band F_t

atoms resulting in a bond order of 0.14 for each F_t‚‚‚F_b

interaction. The lengthening of the Kr-F_bbond and the decrease

of the Kr-F_tbond will lead to a decrease in the actual

interactions involving an individual F_t-Kr- -F_bmoiety and a

decrease in the bond order. There are also fewer electrons

involved in the 5c-6e bond when compared with two 3c-4e

bonds, which is again consistent with a decrease in the Mayer

bond order between F_band F_t. It is useful to note that the total

involvement of F_bwith the two F_tatoms as measured by the

Mayer bond order is 0.28, which is actually greater than that of

an individual fluorine in KrF₂; this is consistent with the 5c-

6e model superimposed on an ionic model in which F^-interacts

+

variations. The Kr- -F_bbridge bond order of Kr₂F₃is 0.38

[0.40] while the Kr-F_tbond order of 0.86 [0.92] is intermediate

with respect to that of free KrF⁺and KrF₂. The valence at the

+

krypton atoms in Kr₂F₃is 1.28 [1.37] and is slightly reduced

with respect to that of KrF₂. The valencies for F_t, 1.04 [1.03],

and F_b, 1.03 [0.94], are also similar, but the fluorine charges

F_t, -0.37 [-0.24], and F_b, -0.16 [-0.46] and Ng atom charges,

0.85 [0.96], indicate that, relative to Xe₂F₃⁺, a significant amount

with two KrF⁺cations. A similar set of results is found for

+ 65

XeF₂and Xe₂F₃

.

The Mayer bond orders are in semiquantitative agreement

with the simple valence bond descriptions of NgF⁺, NgF₂, and

Ng₂F₃⁺, which predict Ng-F bond orders of one-half for NgF₂,

a bond order of one for the free NgF⁺cation, a value somewhere

(61) Mayer, I. Chem. Phys. Lett. 1983, 97, 270.

(62) Mayer, I. Theor. Chim. Acta 1985, 67, 315.

(63) Mayer, I. Int. J. Quantum Chem. 1986, 29, 73.

(64) Mayer, I. Int. J. Quantum Chem. 1986, 29, 477.

(65) Dixon, D. A.; Schrobilgen, G. J. Unpublished results, also see ref 38.

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3015

Table 10. Atomic Charges, Mayer Valencies, and Mayer Bond Orders for [KrF][MF₆] (M ) P, As, Sb, Bi), KrF⁺, Kr₂F₃⁺, and KrF₂

KrF⁺

Kr₂F₃

KrF₂

+

[KrF]‚[PF₆]

[KrF][AsF₆]

[KrF][SbF₆]

[KrF][BiF₆]

Atomic Charges and Valencies^a,b

Kr

M

F_t

F_b

F_a

F_e

F′_e

F′′_e

+0.79 (1.32)

+1.16 (5.36)

-0.30 (0.95)

-0.37 (0.99)

-0.26 (1.14)

-0.26 (1.08)

-0.24 (1.10)

-0.30(1.03)

+0.80 (1.32)

+0.90 (5.60)

-0.27 (0.97)

-0.31 (1.08)

-0.20 (1.20)

-0.22 (1.15)

-0.20 (1.18)

-0.27 (1.10)

+0.82 (1.31)

+1.45 (5.14)

-0.26 (0.98)

-0.31 (1.08)

-0.32 (1.06)

-0.36 (1.00)

+0.82 (1.32)

+2.23 (3.15)

-0.26 (0.98)

-0.41 (0.92)

-0.47 (0.82)

-0.50 (0.76)

-0.46 (0.81)

+1.00 (1.09)

+0.85 (1.28)

+0.72 (1.35)

-0.36 (0.89)

0.00 (1.09)

-0.37 (1.04)

-0.16 (1.03)

Mayer Bond Orders^a

Kr-F_t

Kr-F_b

F_b-F_t

M-F_b

M-F_a

M-F_e

M-F′_e

M-F′′_e

0.74

0.54

0.19

0.23

1.09

1.02

1.03

0.97

0.76

0.50

0.18

0.37

1.10

1.04

1.05

0.98

0.79

0.47

0.17

0.42

0.99

0.96

0.90

0.79

0.48

0.18

0.65

0.61

0.55

1.09

0.86

0.38

0.14

0.67

0.22

^aCalculated values are at the LDFT level. ^bMayer valencies are given in parentheses.

between one-half and one for the terminal Ng-F_tbonds of

structure determination of the low-temperature R-phase. Single-

crystal X-ray structure determinations and theoretical calcula-

tions show that the solid-state and gas-phase [KrF][MF₆] ion

pairs are strongly fluorine bridged. With the exception of

theoretically predicted KrF₂‚PF₅adduct, the Kr-F_tbond length

is little influenced by the MF₆^-anion. No significant difference

Ng₂F₃⁺, and a value between zero and one-half for the Ng-F_b

+

bonds of Ng₂F₃

.

Little variation and no clear pattern is apparent among the

atomic charges of the F_t-Kr- -F_bgroups of the [KrF][MF₆] ion

pairs. The negative charges on the F_aand F_eatoms of the MF₅

groups increase on descending group 15, a trend that is

consistent with a corresponding increase in fluoride ion acceptor

strength (see Computational Results; Geometries. (b) KrF⁺and

[KrF][MF₆]). Thus, the [KrF][MF₆] ion pairs containing the

heavier pnicogens are shown to be more ionic from their M- -

F_bbond orders [0.23 (P), 0.37 (As), 0.42 (Sb), and 0.18 (Bi)],

their valencies at M [5.36 (P), 5.60 (As), 5.14 (Sb), and 3.15

(Bi)], and their Kr- -F_bbond orders [0.54 (P), 0.50 (As), 0.47

(Sb), and 0.48 (Bi)]. The valencies at Kr remain nearly constant

at 1.31-1.32 over the series. The Kr-F_tbond orders [0.74 (P),

0.76 (As), 0.79 (Sb), and 0.79 (Bi)] and F_t- -F_bbond orders

[0.19 (P), 0.18 (As), 0.17 (Sb), and 0.18 (Bi)] are intermediate

-

in the Kr- -F_bbond length was observed in the AsF₆^-and SbF₆

salts, although this bond length was found to be significantly

-

shorter in the BiF₆salt, in accord with the higher covalent

character associated with the weaker fluoride ion acceptor

strength of BiF₅. The Raman spectra of the fluorine-bridged

[KrF][MF₆] ion pairs have been assigned by comparison with

their calculated vibrational spectra.

+

The Kr₂F₃cation is highly deformable in the solid state,

specifically with regards to the Kr- -F_bbond distances and the

Kr- -F_b- -Kr bridge bond angle. These deformations are at-

tributed to long contacts between the krypton atoms in the cation

and fluorine atoms associated with the anion⁵⁰and are not

reproduced by theoretical calculations of the gas-phase geom-

+

with respect to those of Kr₂F₃(0.86 and 0.14, respectively)

+

etries. The calculated vibrational frequencies of Kr₂F₃show

and KrF₂(0.67 and 0.22, respectively). The most significant

increase in M- -F_bbond order and in pnicogen valence occurs

on going from P to As with a smaller decrease in the Kr- -F_b

bond order. Similar changes are noted for the transition from

As to Sb, but no significant changes in the aforementioned

parameters occur in going from Sb to Bi except that the Bi- -F_b

bond order is smaller by 0.24. The anomaly seen for bismuth

likely arises from the core potential used in the calculations.

These results show that the degree of F^-complexation increases

as the size of the central atom increases, and the coordination

sphere around the pnicogen atom becomes less crowded,

allowing MF₅to more effectively compete with the KrF⁺cation

for the bridge fluorine. This is consistent with the gas-phase

fluoride ion affinities of PF₅, AsF₅, and SbF₅⁵⁷and our failure

to prepare [KrF][PF₆] (see Synthesis and Characterization by

Raman Spectroscopy of [Kr₂F₃][PF₆]‚nKrF₂). Moreover, a

comparison of the P- -F_bbond order in [KrF][PF₆] with the Kr- -

that the prior detailed assignments¹⁶of the terminal symmetric

and asymmetric stretching modes were incorrect and that their

assignments should be interchanged.

Raman spectroscopic investigation of the KrF₂/PF₅system

indicates that the only existing species is [Kr₂F₃][PF₆]‚nKrF₂.

This compound is characteristic of the [Kr₂F₃][MF₆]‚nKrF₂(M

) As, Sb, Bi) series and exhibits vibrational modes consistent

-

with a weakly interacting Kr₂F₃⁺cation and an octahedral PF₆

anion. Failure to synthesize [KrF][PF₆] in the presence of excess

PF₅reflects the low fluoride ion affinity of PF₅relative to those

of the heavier pnicogen pentafluorides and conforms with the

theoretical results, which predict an adduct in which PF₅is

weakly fluorine bridged to KrF₂and the KrF₂bond lengths

exhibit little distortion from the bond lengths of free KrF₂.

Experimental Section

F_bbond order in Kr₂F₃indicates that KrF⁺has a greater F^-

+

Caution: Anhydrous HF must be handled using appropriate

protective gear with immediate access to proper treatment procedures

in the event of contact with liquid HF, HF vapor, or HF solutions of

krypton(II) species. Krypton(II) compounds are potent oxidizers that

react explosively with water and organic materials. Extreme caution is

to be exercised in their handling and disposal in order to avoid violent

detonations. The recommended procedure for disposing of small

affinity than PF₅. Our inability to synthesize [KrF][PF₆] at low

temperatures in the presence of excess PF₅is consistent with

these findings.

Conclusion

+

quantities of KrF₂, KrF⁺, and Kr₂F₃salts in anhydrous HF solvent

A more precise structure of KrF₂has been obtained, and the

dimorphism of KrF₂has been confirmed by an X-ray crystal

(ca. 200 mg of compound in 2 mL of HF or less) is by slowly pouring

3016 Inorganic Chemistry, Vol. 40, No. 13, 2001

Lehmann et al.

the cold HF solution (0 °C or lower) into several liters of a mixture of

ice and aqueous NaOH inside a properly shielded fume hood. Solution

sample containers must be handled with long metal tongs and protective

gloves when dumping their contents into the disposal medium.

Apparatus and Materials. Volatile materials were handled in

vacuum lines constructed of stainless steel, nickel, and FEP fluoro-

plastic, and nonvolatile materials were handled in the dry nitrogen

atmosphere of a glovebox as previously described.⁶⁶

Arsenic pentafluoride was prepared as previously described⁶⁷and

was used without further purification. Anhydrous HF (Harshaw

Chemical Co.),⁶⁸SbF₃(Aldrich, 98%),⁶⁶and BiF₅(Ozark Mahoning

Co.)¹⁷were purified by the standard literature methods. Purified HF

was stored over BiF₅in a Kel-F vessel equipped with a Kel-F valve

until used. Fluorine gas (Air Products) was used without further

purification. Antimony pentafluoride was synthesized in situ by direct

fluorination of SbF₃with F₂in anhydrous HF as previously described.⁶⁹

(a) PF₅. Phosphorus pentafluoride (Ozark Mahoning) was purified

by distillation under dynamic vacuum through two FEP traps maintained

at -117 °C to remove POF₃and HF impurities and at -196 °C to

condense PF₅. This process was repeated a second time before the

purified product was condensed into a prefluorinated stainless steel

cylinder at -196 °C. The cylinder and contents were stored at room

temperature until used.

between -78 and -53 °C with agitation for 1 h. Excess AsF₅was

then removed under dynamic vacuum at -78 °C. Raman spectroscopy

at -78 °C confirmed the presence of both R-[KrF][AsF₆] and â-[KrF]-

[AsF₆] in the sample.¹⁶The [Kr₂F₃][AsF₆]‚[KrF][AsF₆] adduct was

synthesized by condensing a slight stoichiometric excess of KrF₂onto

a sample of [KrF][AsF₆]. Sufficient anhydrous HF (ca. 1.5 mL) was

condensed into each reactor until the sample was completely soluble

at -10 °C. The reactor was then backfilled with ultra-high-purity (UHP)

nitrogen (99.999%, <10 ppm O₂+ CO + CO₂+ H₂O) or argon

(99.999%, <2 ppm O₂, <3 ppm H₂O) and stored at -78 °C.

[KrF][SbF₆] and [KrF][BiF₆]. The salts, [KrF][SbF₆] and [KrF]-

[BiF₆], were prepared by the literature methods.^16,17In each preparation,

approximately 100 mg of the pentafluoride was dissolved in 1-2 mL

of anhydrous HF in a rigorously dried and prefluorinated T-shaped

1

reaction vessel fabricated from /₄-in. o.d. FEP tubing and fitted with

a stainless steel valve. A stoichiometric excess of KrF₂(ca. 10% to

account for the passivation of the stainless steel valve by KrF₂) was

then condensed on top of the mixture at -196 °C before being warmed

to -78 °C and backfilling the vessel with UHP nitrogen or argon. The

vessel was then carefully warmed to 0 °C and agitated for 10 min before

storing at -78 °C.

[Kr₂F₃][PF₆]‚nKrF₂. Approximately 100 mg of KrF₂was vacuum-

distilled into a dried and F₂-passivated FEP reaction vessel equipped

with a stainless steel valve. Phosphorus pentafluoride (bp, -75 °C)

was then condensed into the reaction vessel and allowed to react at

-78 °C. The reaction was monitored biweekly by Raman spectroscopy,

and additional aliquots of PF₅were added as necessary. The reaction

was determined to be complete when the changes in the Raman

spectrum ceased after 7 weeks, and the intense ν₁(Σ⁺_g) vibration of

KrF₂was barely visible at 465 cm^-1. Further reaction with PF₅failed

to produce [Kr₂F₃][PF₆] or [KrF][PF₆]. The spectrum was also monitored

for [O₂][PF₆] arising from the reaction of Kr₂F₃⁺with O₂or H₂O. Only

extremely weak peaks associated with the O₂⁺stretches at 1859.5 and

1863.0 cm^-1were observed after 7 weeks.

(b) KrF₂. Krypton difluoride was prepared using a 316 stainless

steel hot-wire reactor equipped with a nickel filament, similar to that

originally described by Bezmel’nitsyn et al.⁷and subsequently modified

and described by Kinkead et al.⁹The filament of the reactor used in

the present work was fabricated from ¹/₁₆-in. nickel rod tightly wound

1

about a second length of /₁₆-in. rod that was, in turn, coiled and

stretched into a helix. Approximately 1000 Torr (50 mmol) of krypton

(Air Products, 99.995%) was initially added to the prefluorinated

reactor, which was then immersed in a 20 L dewar filled with liquid

nitrogen. After the reactor was cooled, 25 Torr of F₂(Air Products,

technical grade) was added, and the DC power supply for the nickel

filament was adjusted to approximately 6 V and 30 A (the filament

under these conditions was dull red in color), which increased the F₂

pressure to ca. 45 Torr once the temperature of the filament and reactor

had stabilized. The pressure inside the hot-wire reactor was maintained

between 25 and 45 Torr for each 8-12 h run. As KrF₂formation

progressed, the pressure was periodically readjusted by adding more

F₂. When KrF₂production slowed or ceased, a 1.0-2.0 mmol aliquot

of krypton was condensed into the reactor. After the reaction was

completed, excess fluorine was removed by pumping the reactor at

-196 °C. Excess krypton and crude KrF₂were recovered as a pink

solid (because of chromium oxide fluoride contamination leached from

the stainless steel reactor body) by allowing the reactor to slowly warm

to room temperature while dynamically pumping the volatile contents

[Kr₂F₃][SbF₆]‚KrF₂and [Kr₂F₃]₂[SbF₆]₂‚KrF₂. The [Kr₂F₃][SbF₆]‚

KrF₂and [Kr₂F₃]₂[SbF₆]₂‚KrF₂salts were synthesized in the same

fashion as the [KrF][SbF₆] salt with only minor modifications. The

[Kr₂F₃][SbF₆]‚KrF₂salt was synthesized using a large excess of KrF₂

(3.5KrF₂:SbF₅, a 75 mol % excess with respect to Kr₂F₃⁺). The [Kr₂F₃]₂-

[SbF₆]₂‚KrF₂salt was synthesized using a 0.6 mol % stoichiometric

excess of KrF₂. After both samples were crystallized, both [Kr₂F₃]₂-

[SbF₆]₂‚KrF₂and [KrF][SbF₆] were shown to be present by determina-

tions of their respective unit cell parameters.

Crystal Growth. r-KrF₂. Approximately 50 mg of KrF₂was

distilled into one arm of an h-shaped reaction vessel constructed of

FEP and equipped with a Kel-F valve. The reaction vessel was

evacuated at -78 °C on a metal vacuum line and then allowed to stand

under static vacuum. The arm containing KrF₂was warmed from -78

to -40 °C using a dry ice/acetone bath, while the other arm of the

reactor was maintained at -78 °C. Over the course of several hours,

nearly cubic crystals of KrF₂were deposited in the colder arm above

the coolant level. The reactor was backfilled with UHP N₂when the

sublimation was complete and stored at -78 °C until the crystals could

be mounted on the X-ray diffractometer.

1

through a /₂-in. o.d. FEP U-trap that was cooled to -196 °C. The

Kr/KrF₂mixture was then warmed and dynamically pumped at -78

°C to remove unreacted krypton. The crude KrF₂was purified by briefly

warming the sample to approximately 0 °C and flash distilling off the

more volatile chromium oxide fluorides. The remaining colorless KrF₂

was finally warmed to room temperature and rapidly sublimed into a

³/₈-in. o.d. FEP tube equipped with a Kel-F valve and stored under

nitrogen or argon at -78 °C until used. This synthesis is highly

reproducible and typically yields 2.5-3.0 g of purified KrF₂over a 12

h period.

[KrF][MF₆] (M ) As, Sb, Bi), [Kr₂F₃]₂[MF₆]₂‚KrF₂, and [Kr₂F₃]-

+

[MF₆]‚KrF₂(M ) As, Sb). The Kr₂F₃salts and their adducts with

KrF₂were completely dissolved, or nearly completely in the case of

the moderately soluble [KrF][BiF₆] salt, by carefully warming the

solutions to a maximum of -10 °C to minimize decomposition. The

reaction vessel was then slowly cooled by insertion into a glass,

vacuum-jacketed tube that was cooled by a flow of cold nitrogen gas.

The rate of cooling was controlled by the nitrogen gas flow rate and

could be regulated ((0.2 °C) between room temperature and -90 °C.

Once cooled to -80 °C, the bulk of the HF was decanted into the

sidearm of the T-shaped FEP reactor, which was cooled to -196 °C

immediately before the solvent was decanted. The sample was then

pumped on dynamically to thoroughly dry the crystals before the

sidearm was heat sealed off. The reactor was then backfilled with dry

N₂and stored at -78 °C until the crystals could be mounted on the

X-ray diffractometer. The [KrF][MF₆] (M ) As, Sb, Bi) salts all

â-[KrF][AsF₆] and [Kr₂F₃][AsF₆]‚[KrF][AsF₆]. The salts [Kr₂F₃]-

[AsF₆]‚[KrF][AsF₆] and [KrF][AsF₆] were prepared in a manner similar

to that described earlier for [KrF][AsF₆].¹⁶Approximately 100 mg of

KrF₂was condensed at -196 °C into a ¹/₄-in. o.d. T-shaped FEP reactor

equipped with a stainless steel valve. An excess of AsF₅was then

condensed on top of the KrF₂, and the reaction vessel was cycled

(66) Casteel, W. J., Jr.; Kolb, P.; LeBlond, N.; Mercier, H. P. A.;

Schrobilgen, G. J. Inorg. Chem. 1996, 35, 929.

(67) Mercier, H. P. A.; Sanders, J. C. P.; Schrobilgen, G. J.; Tsai, S. S.

Inorg. Chem. 1993, 32, 386.

(68) Emara, A. A. A.; Schrobilgen, G. J. Inorg. Chem. 1992, 31, 1323.

(69) LeBlond, N.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2000,

39, 2473.

X-ray Crystal Structures

Inorganic Chemistry, Vol. 40, No. 13, 2001 3017

crystallized as discreet colorless needles, while [Kr₂F₃][SbF₆]‚KrF₂and

[Kr₂F₃]₂[SbF₆]₂‚KrF₂crystallized as intergrown pale yellow plates.

Raman Spectroscopy. Raman spectra of [Kr₂F₃][PF₆]‚nKrF₂were

recorded as previously described.⁶⁶The spectra were recorded in a ¹/₄-

in. FEP sample tube under 1000 Torr of PF₅at -80 C° using the

macrochamber of the instrument. The 514.5-nm line of an Ar⁺laser

with an output setting of 300 mW was used for excitation of the sample

with a laser spot of ∼1 mm at the sample. The slit width settings

corresponded to a resolution of 1 cm^-1. A total of 10 reads each having

60 s integration times were summed using a CCD detector.

X-ray Crystallography. Crystal Mounting and the Collection of

X-ray Data. All crystals were mounted at -110(5 °C as previously

described,⁷⁰and data were collected at -113 to -130 °C. All crystals

were centered on a P4 Siemens diffractometer, equipped with a Siemens

SMART 1K CCD area detector, controlled by SMART,⁷¹and a rotating

anode emitting KR radiation monochromated (λ ) 0.710 73 Å) by a

graphite crystal. The crystal to detector distance was typically 5 cm,

and the data collection was carried out in a 512 × 512 pixel mode

using 2 × 2 pixel binning. Processing of the raw data was completed

using SAINT+,⁷²which applied Lorentz and polarization corrections

to the three-dimensionally integrated diffraction spots. Scaling of the

integrated data was completed with SADABS,⁷³which applies decay

corrections, and an empirical absorption correction on the basis of the

intensity ratios of redundant reflections.

sets in UniChem.⁸³All calculations were done at the local level with

the potential fit of Vosko, Wilk, and Nusair.⁸⁴The geometries were

optimized by using analytic gradient methods, and second derivatives

were also calculated analytically.⁸⁵The HF calculations were done with

the program Gaussian-98.⁸⁶For the F and P atoms, the polarized

double-ú basis sets of Dunning and Hay were used.⁸⁷For the remaining

atoms, the effective core potentials and double-ú basis sets of Hay and

Wadt^80-82were used with the polarization functions of Huzinaga et

al.⁸⁸

Acknowledgment. We thank the donors of the Petroleum

Research Fund, administered by the American Chemical Society,

for support of this work under ACS-PRF 33594-AC3. We also

thank the Natural Sciences and Engineering Research Council

of Canada for a postgraduate scholarship and McMaster

University for a Dalley Fellowship to J.F.L. and the Canada

Council for the award of a Killam Research Fellowship (1998

and 1999) to G.J.S. The density-functional theory calculations

were performed in the William R. Wiley Environmental

Molecular Sciences Laboratory funded by the Office of Biologi-

cal Environmental Research, U.S. Department of Energy, under

Contract DE-AC06-76RLO 1830 with the Battelle Memorial

Institute, which operates the Pacific Northwest National Labora-

tory, a multiprogram national laboratory for the Department of

Energy.

Solution and Refinement of the Structures. The program XPREP⁷⁴

was used to confirm the unit cell dimensions and the crystal lattice. A

solution was found using direct methods to determine the locations of

the heavy elements (Kr, As, Sb, Bi). The fluorine positions were

identified in successive difference Fourier syntheses. Final refinements

were obtained using data that had been corrected for absorption by

introducing an extinction coefficient and optimized using anisotropic

thermal parameters.

Supporting Information Available: An X-ray crystallographic file,

in CIF format, for the structure determinations of R-KrF₂, [KrF][MF₆]

(M ) As, Sb, Bi), [Kr₂F₃][SbF₆]‚KrF₂, [Kr₂F₃]₂[SbF₆]₂‚KrF₂, and [Kr₂F₃]-

[AsF₆]‚[KrF][MF₆]. This material is available free of charge via the

Internet at http://pubs.acs.org.

Calculations. Electron structure calculations at the DFT level were

done with the density-functional theory⁷⁵program DGauss^76-78on SGI

computer systems. The DZVP basis set⁷⁹was used for all atoms, except

for Bi where the Hay-Wadt ECP basis set was used^80-82with the fitting

IC001167W

(80) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1982, 82, 271.

(81) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1982, 82, 285.

(82) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1982, 82, 299.

(83) Lee, C.; Chen, H. Unpublished results; see UniChem Manual, Version

3.0.

(70) Gerken, M.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2000, 39,

4244.

(71) SMART, Version 5.054; Siemens Energy and Automation Inc.:

Madison, WI, 1999.

(84) Vosko, S. J.; Wilk, L.; Nusair, W. Can. J. Phys. 1980, 58, 1200.

(85) Komornicki, A.; Fitzgerald, G. J. J. Chem. Phys. 1993, 98, 1398.

(86) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery J. A., Jr.;

Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,

A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,

V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;

Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;

Morokuma K.; Malick D. K.; Rabuck A. D.; Raghavachari, K.;

Foresman, J. B.; Ciosloski, J.; Ortiz, J. V.; Stefanov, B. B.; Lui, G.;

Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.

L.; Fox, D. J.; Keith T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,

A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,

W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;

Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.4; Gaussian,

Inc.: Pittsburgh, PA, 1998.

(72) SAINT+, Version 6.01; Siemens Energy and Automation Inc.:

Madison, WI, 1999.

(73) Sheldrick, G. M. SADABS (Siemens Area Detector Absorption

Corrections), Personal Communication, 1998.

(74) Sheldrick, G. M. SHELXTL, Version 5.1; Siemens Analytical X-ray

Instruments Inc.: 1998.

(75) Par, R. G.; Yang, W. Density-Functional Theory of Atoms and

Molecules; Oxford University Press: New York, 1989.

(76) Andzelm, J.; Wimmer, E.; Salahub, D. R. In The Challenge of d and

f Electrons: Theory and Computation; Salahub, D. R., Zerner, M.

C., Eds.; ACS Symposium Series 394; American Chemical Society:

Washington, DC, 1989; p 228.

(77) Andzelm, J. In Density Functional Theory in Chemistry; Labanowski,

J., Andzelm, J., Eds.; Springer-Verlag: New York, 1991; p 155.

(78) Andzelm, J. W.; Wimmer, E. J. J. Chem. Phys. 1992, 96, 1280. DGauss

is a density-functional program that is part of UniChem and is available

from Oxford Molecular. Versions 4.1 and 5.0 were used.

(79) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem.

1992, 70, 560.

(87) Dunning, T. H., Jr.; Hay, P. J. In Methods of Electronic Structure

Theory; Schaefer, H. F., III, Ed.; Plenum: New York, 1977; p 1.

(88) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.;

Sakai, Y.; Tatwaki, H. Gaussian Basis Sets for Molecular Calculations;

Physical Sciences Data 16; Elsevier: Amsterdam, 1984.

Article Doi

X-ray crystal structures of α-KrF2, [KrF][MF6] (M = As, Sb, Bi), [Kr2F3][SbF6]·KrF2, [Kr2F3]2[SbF6] 2·KrF2, and [Kr2F3][AsF6]·[KrF][AsF6];

DOI: 10.1021/ic001167w

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic001167w

Full text of DOI:10.1021/ic001167w

Products guided by the article

R&D Labs maybe for 13773-81-4

Relevant to this article

Hot Product