Seven-coordinated pnicogens. Synthesis and characterization of the SbF72- and BiF72- dianions and a theoretical study of the AsF72- dianion

Article abstract of DOI:10.1021/ja9805728

The novel seven-coordinated BiF₇^2- and SbF₇^2- dianions have been prepared and characterized. The Cs₂BiF₇, Rb₂BiF₇, K₂BiF₇, and Na₂BiF₇ salts were obtained in high yield by heating BiF₅ with an excess of the corresponding alkali metal fluorides to about 250 °C. Attempts failed to prepare the corresponding BiF₈^3- salts or Li₂BiF₇ under similar conditions. The [N(CH₃)₄]2BiF₇ salt was obtained by the combination of excess N(CH₃)₄F with BiF₅ in CH₃CN solution at -31 °C. The (NO₂)₂BiF₇ salt was prepared from BiF₅ and a large excess of liquid FNO at -78 °C and decomposes at room temperature to NOBiF₆ and FNO. The corresponding Cs₂SbF₇, K₂SbF₇, and [N(CH₃)₄]₂SbF₇ salts were also synthesized in a similar fashion, but Na₂SbF₇ was not formed. The pronounced fluoride ion affinity of SbF₆^- was further demonstrated by the formation of some Cs₂SbF₇ when dry CsF and CsSbF₆ were ball-milled at room temperature. The BiF₇^2- and SbF₇^2- anions, which are the first examples of binary pnicogen compounds with coordination numbers in excess of six, were characterized by vibrational spectroscopy and ab initio electronic structure calculations. They possess pentagonal bipyramidal, highly fluxional structures of D(5h) symmetry, similar to those of IF₇ and TeF₇^-, which are isoelectronic with SbF₇^2-. Although our theoretical calculations indicate that AsF₇^2- is also vibrationally stable, experiments to prepare this dianion were unsuccessful.

Full text of DOI:10.1021/ja9805728

8392
J. Am. Chem. Soc. 1998, 120, 8392-8400
Seven-Coordinated Pnicogens. Synthesis and Characterization of the
2-
2-
2-
SbF7 and BiF7 Dianions and a Theoretical Study of the AsF7
Dianion
Greg W. Drake,*,† David A. Dixon,‡ Jeffrey A. Sheehy,† Jerry A. Boatz,† and
Karl O. Christe*,§,|
Contribution from Raytheon STX and the Propulsion Directorate, Air Force Research Laboratory,
Edwards Air Force Base, California 93524, Loker Hydrocarbon Research Institute, UniVersity of Southern
California, Los Angeles, California 90089, and Pacific Northwest National Laboratory,
Richland, Washington 99352
ReceiVed February 19, 1998
2-
2-
Abstract: The novel seven-coordinated BiF7 and SbF7 dianions have been prepared and characterized.
The Cs2BiF7, Rb2BiF7, K2BiF7, and Na2BiF7 salts were obtained in high yield by heating BiF5 with an excess
of the corresponding alkali metal fluorides to about 250 °C. Attempts failed to prepare the corresponding
3-
BiF8 salts or Li2BiF7 under similar conditions. The [N(CH3)4]2BiF7 salt was obtained by the combination
of excess N(CH3)4F with BiF5 in CH3CN solution at -31 °C. The (NO2)2BiF7 salt was prepared from BiF5
and a large excess of liquid FNO at -78 °C and decomposes at room temperature to NOBiF6 and FNO. The
corresponding Cs2SbF7, K2SbF7, and [N(CH3)4]2SbF7 salts were also synthesized in a similar fashion, but
-
Na2SbF7 was not formed. The pronounced fluoride ion affinity of SbF6 was further demonstrated by the
formation of some Cs2SbF7 when dry CsF and CsSbF6 were ball-milled at room temperature. The BiF72- and
2-
SbF7 anions, which are the first examples of binary pnicogen compounds with coordination numbers in
excess of six, were characterized by vibrational spectroscopy and ab initio electronic structure calculations.
They possess pentagonal bipyramidal, highly fluxional structures of D5h symmetry, similar to those of IF7 and
TeF7-, which are isoelectronic with SbF72-. Although our theoretical calculations indicate that AsF72- is also
vibrationally stable, experiments to prepare this dianion were unsuccessful.
Introduction
only one but also two F- ions with formation of the TeF7- and
TeF8 anions, respectively,7-10 suggested that isoelectronic
2-
Except for cluster compounds, such as [Rh10As(CO)22]3- and
[Rh12Sb(CO)27]3-, in which the pnicogen atoms are located
inside a cluster cavity,1,2 the maximum coordination numbers
-
SbF6 might also possess sufficient Lewis acidity to add a
second F- ion and form a stable SbF72- dianion. Therefore, it
was interesting to explore whether the well-known heavier
(CN) reported for binary antimony and bismuth compounds, as
- 11,12
- 12,13
- 14
hexafluoropnictate anions, AsF6
,
SbF6
,
and BiF6 ,
3-
a rule, do not exceed six. Even in the triply charged SbCl6
,
could bind additional F- ligands and form multiply charged
anions with coordination numbers higher than six.
SbBr63-, and BiCl6
anions, in which the central atom
3-
possesses, in addition to the six halogen ligands, a free valence
electron pair, the latter is sterically inactive, and CN 6 is not
exceeded.3-5 CN 6 had been generally accepted as the coordina-
tion limit for the pnicogens, and it appears that, probably due
to the “magic” stability traditionally attributed to the sp3 and
sp3d2 valence electron shells, no systematic efforts have been
made to expand the 12 valence electron shells in the hexafluo-
ropnictate(V) anions. However, the relatively high F- ion
affinity of TeF6 (∼80 kcal mol-1)6 and its ability to bind not
Experimental Section
Materials and Methods. The synthesis of anhydrous N(CH3)4F
has been previously described.15 The alkali metal fluorides were fused
in platinum crucibles, transferred while hot to the drybox, and finely
powdered. Antimony pentafluoride and AsF5 (Ozark Mahoning Co.)
were purified by fractional condensation. Bismuth pentafluoride (Ozark
Mahoning Co.) did not contain any detectable impurities and was used
as received. Acetonitrile (Baker, bioanalyzed, with a water content of
less than 40 ppm) was treated with P2O5 and freshly distilled prior to
† Propulsion Directorate, Air Force Research Laboratory.
‡ Pacific Northwest National Laboratory.
(7) Muetterties, E. L. J. Am. Chem. Soc. 1957, 79, 1004.
(8) Selig, H.; Sarig, S.; Abramowitz, S. Inorg. Chem. 1974, 13, 1508.
(9) Christe, K. O.; Sanders, J. C. P.; Schrobilgen, G.; Wilson, W. W. J.
Chem. Soc., Chem. Commun. 1991, 837.
(10) Zhang, X.; Seppelt, K. Z. Anorg. Allg. Chem. 1997, 623, 491.
(11) Weidlein, J.; Dehnicke, K. Z. Anorg. Allg. Chem. 1965, 337, 113.
(12) Christe, K. O.; Maya, W. Inorg. Chem. 1969, 8, 1253.
(13) Begun, G. M.; Rutenberg, A. C. Inorg. Chem. 1967, 6, 2212.
(14) Christe, K. O.; Wilson, W. W.; Schack, C. J. J. Fluorine Chem.
1978, 11, 71. Christe K. O.; Wilson, W. W.; Schack, C. J.; Bougon, R. J.
Fluorine Chem. 1978, 12, 336.
§ Raytheon STX.
| University of Southern California.
(1) Vidal, J. L. Inorg. Chem. 1981, 20, 243.
(2) Vidal, J. L.; Troup, J. M. J. Organomet. Chem. 1981, 213, 351.
(3) Gillespie, R. J.; Hargittai, I. The VESPR Model of Molecular
Geometry; Allyn and Bacon: Boston, 1991.
(4) Klapo¨tke, T. M.; Torrieporth-Oetting, I. C. Nichtmetallchemie;
VSH: Weinheim, Germany, 1994.
(5) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon Press: Oxford, 1984.
(6) Dixon, D. A.; Christe, K. O. Unpublished results based on ab initio
calculations.
(15) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng, J. J.
Am. Chem. Soc. 1990, 112, 7619.
S0002-7863(98)00572-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/08/1998

SeVen-Coordinated Pnicogens
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8393
use. The F2 (Air Products) was passed through a NaF scrubber for the
removal of HF. Nitrosyl fluoride was prepared from NO and F2 at
-196 °C in a Teflon-FEP U-trap and purified by fractional condensa-
tion.16
in a dynamic vacuum, the solid lost half of its FNO content, leaving
behind NOBiF6 [Ra at 25 °C: NO+, ν ) 2338(5); BiF6-, ν1(A1g) )
590(100), ν2(Eg) ) 547(3) and 519(11), ν5(F2g) ) 247(13), 231(10)
and 212(2)] and unreacted BiF5.
Preparation of M2SbF7 (M ) K, Cs, N(CH3)4). The procedures
and reaction conditions were analogous to those described above for
M2BiF7, except for using SbF5 in place of BiF5. With a mole ratio of
MF:SbF5 of 3:1 and heating for 4 days to 250 °C, the mole ratio of
Acetonitrile was transferred in a flamed-out Pyrex glass vacuum line
that was equipped with Kontex glass-Teflon valves and a Heise
pressure gauge. The oxidizers were handled in a stainless steel vacuum
line equipped with Teflon-FEP U-traps, 316 stainless steel bellows
seal valves, and a Heise pressure gauge.17 Solids were handled in the
dry nitrogen atomosphere of a glovebox.
Infrared spectra were recorded on either a Perkin-Elmer model 283
or a Mattson Galaxy spectrometer using AgBr disks prepared by
pressing the finely powdered sample between two thin AgBr plates in
a Barnes Engineering minipress inside the glovebox. Raman spectra
were recorded on either a Cary model 83 GT or a Spex model 1403
spectrophotometer using the 488-nm exciting line of an Ar ion or the
647.1-nm line of a Kr ion laser, respectively. A previously described18
device was used for the recording of the low-temperature spectra.
-
SbF72-:SbF6 in the product was about 3:1. However, heating of a
mixture of CsSbF6 and CsF in a mole ratio of 1:2 to 300 °C for 45 h
resulted in essentially complete conversion of CsSbF6 to Cs2SbF7.
A mixture of CsSbF6 and CsF in a mole ratio of 1:2, together with
two steel balls, was placed into the stainless steel cup of a high-
frequency “Wig-L-Bug” shaker, normally used for the preparation of
samples for infrared spectroscopy. The Raman spectrum of the mixture
after 1 h of vigorous shaking at room temperature showed a conversion
-
2-
of SbF6 to SbF7 of about 13%.
Reactions of Cs2XF7 (X ) Bi, Sb) with Anhydrous HF. Samples
of 1:1 mixtures of Cs2BiF7/CsF or Cs2SbF7/CsF, when treated with a
large excess of anhydrous HF at room temperature, were converted to
CsXF6 and 2CsHF2, as shown by mass balance and vibrational
spectroscopy.
Preparation of M2BiF7 (M ) Na, K, Rb, Cs). In a typical
experiment, finely powdered dry KF (21.0 mmol) and BiF5 (7.0 mmol)
were loaded inside the drybox into a prepassivated (with ClF3) 75-mL
Monel reactor which was closed by a Monel valve. On the metal
vacuum line, the reactor was evacuated, and gaseous fluorine (300 Torr)
was added. The reactor was heated in an electrical oven to 250 °C for
4 days. The reactor was removed from the oven, and the fluorine gas
was pumped off. The white solid product (3349 mg, mass calcd for
7.0 mmol of K2BiF7 and 7.0 mmol of KF ) 3348 mg) was shown by
vibrational spectroscopy to be a mixture of K2BiF7 and KF and did
not contain any amounts of KBiF6 or unreacted BiF5, detectable by
Raman spectroscopy. Using only a 2:1 mole ratio of MF:BiF5 and
Computational Methods. Quantum-chemical calculations employ-
ing the Hartree-Fock (HF) self-consistent-field and Møller-Plesset
second-order perturbation theory (MP2) methods were performed for
2-
the free D5h symmetry BiF7 compound in several atomic basis sets,
yielding equilibrium structures and vibrational spectra. To calibrate
the methods and basis sets, analogous calculations were carried out on
the well-characterized and closely related octahedral BiF6- anion. Basis
set 1 was comprised of a double-ú plus polarization (DZP) basis on
fluorine atoms,21 with DZP basis sets for the valence shells and effective
core potentials (ECP) for the inner shells of bismuth.22 Basis set 2
consisted of the 6-311G(d) fluroine basis23 and the LANL2DZ22,24
bismuth ECP/valence-DZ basis supplemented with a d function having
an exponent of 0.185 on bismuth. Basis set 3 was comprised of the
SBKJC valence-double-ú and ECP sets for both fluorine and bismuth
atoms,24-26 supplemented with d functions having exponents of 0.80
and 0.185, respectively, and with diffuse s and p functions with common
exponents of 0.1076 and 0.0215, respectively. Basis set 4 was simply
basis 2 supplemented with diffuse s and p functions having common
exponents of 0.1288 and 0.0515 for fluorine and 0.0298 for bismuth.
And finally, basis set 5, comprised from elements of sets 3 and 4, can
be succinctly summarized as F:6-311G(d; sp ) 0.1288, 0.0515)/Bi:
2-
lower reaction temperatures and shorter reaction times, BiF7 salts
were still the major products but frequently contained some MBiF6
impurities. For the synthesis of Cs2BiF7, a 3:1 mole ratio of CsF:BiF5
and a reaction temperature of 310 °C for 4 days gave a product that
-
was essentially free of BiF6
.
Preparation of [N(CH3)4]2BiF7. In the drybox, a prepassivated
(with ClF3) 3/4-in.-o.d. Teflon-FEP ampule, which was equipped with
a Teflon-coated magnetic stirring bar and was closed by a stainless
steel valve, was loaded with anhydrous N(CH3)4F (1.93 mmol). On
the glass vacuum line, dry CH3CN (5.754 g) was added at -196 °C,
and the mixture was warmed to room temperature to dissolve the
N(CH3)4F. The ampule was cooled to -196 °C, and BiF5 (0.961 mmol)
was added to the cold ampule inside the drybox. The ampule was
reconnected to the glass line and warmed to -31 °C for 100 min with
stirring. Upon melting of the CH3CN, a yellow color was observed,
followed by the formation of a light yellow precipitate. The CH3CN
solvent was pumped off at -22 °C in a dynamic vacuum, leaving behind
499 mg (mass calcd for 0.961 mmol of [N(CH3)4]2BiF7 ) 471 mg) of
a light yellow solid which, based on its vibrational spectra, contained
-
SBJKC(d ) 0.185; sp ) 0.01857). Based on results obtained for BiF6
and BiF72-, basis sets 1, 3, and 5 were chosen for use in HF and MP2
calculations on the analogous arsenic and antimony compounds.
Harmonic vibrational frequencies and infrared intensities were
computed on the basis of analytic first and numerical second derivatives
of the molecular energy and first derivatives of the dipole moment with
respect to nuclear coordinates at all levels of calculation. Calculations
employed the GRADSCF,27 GAMESS,28 and Gaussian29 program
systems. The calculated Hessian matrixes (second derivatives of the
2-
+
mainly the BiF7 and N(CH3)4 ions,15,19 in addition to some small
amounts of BiF6- and some HF2- and H2F3
the extra weight.
,
- 19 which can account for
Preparation of (NO)2BiF7. In a typical experiment, BiF5 (1.618
mmol) was loaded in the drybox into a prepassivated 3/4-in.-o.d. Teflon-
FEP ampule which contained a Teflon-coated magnetic stirring bar and
was closed with a stainless steel valve. On the metal vacuum line,
FNO (23.83 mmol) was added at -196 °C, and the mixture was stirred
for 24 h at -78 °C. The excess of unreacted FNO (21 mmol) was
pumped off at -78 °C, leaving behind a white solid which was shown
by low-temperature Raman spectroscopy to be a mixture of (NO2)-
BiF7 and unreacted BiF5 [596(100), 569(10), 252(8), 165(7)br, in good
agreement with a previous report20]. On warming to room temperature
(21) Dunning, T. H., Jr.; Hay, P. J. In Methods of Electronic Structure
Theory; Schaefer, H. F., III, Ed.; Plenum: New York, 1977; Chapter 1.
(22) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
(23) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650.
(24) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984, 81,
6026.
(25) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 612.
(26) Basis sets and some of the terminology used here were obtained
from the Gaussian Basis Set Library, developed and distributed by the
Environmental and Molecular Sciences Laboratory of Pacific Northwest
National Laboratory, accessible on the World Wide Web at universal
resource locator http://www.emsl.pnl.gov: 2080/forms/basisform.html.
(27) GRADSCF is an ab initio quantum chemistry program system
designed and written by A. Komornicki, Polyatomics Research Institute,
Redwood City, CA.
(28) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347.
(16) Christe, K. O.; Schack, C. J. Inorg. Chem. 1970, 9, 1852.
(17) Christe, K. O.; Wilson R. D.; Schack, C. J. Inorg. Synth. 1986, 24,
3.
(18) Miller, F. A.; Harney, B. M. Appl. Spectrosc. 1969, 23, 8.
(19) Wilson, W. W.; Christe, K. O.; Feng, J.; Bau, R. Can. J. Chem.
1989, 67, 1898.
(20) Beattie, I. R.; Livingston, K. M. S.; Ozin, G. A.; Reynolds, D. J. J.
Chem. Soc. (A) 1969, 958. Beattie, I. R.; Cheetham, N.; Gilson, T. R.;
Livingston, K. M. S.; Reynolds, D. J. J. Chem. Soc. (A) 1971, 1910.

8394 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Drake et al.
Figure 1. Raman spectra of solid Na2BiF7, K2BiF7, Rb2BiF7, and Cs2-
BiF7, recorded at 25 and -130 °C. Bands masked by an asterisk are
due to BiF6-. The frequencies are listed in Table 1.
Figure 3. Raman spectra of (NO)2BiF7 recorded at -130 °C. (A)
Spectrum of mainly (NO)2BiF7 containing a small amount of BiF5,
which is marked by asterisks. (B) Spectrum of mainly BiF5 containing
a small amount of (NO)2BiF7, which is marked by a circle.
Figure 2. Room-temperature infrared spectra of pressed AgBr disks
of the following solids: (A) CsF; (B) CsBiF6; (C) Cs2BiF7, containing
small amounts of CsBiF6 and CsF; and (D) Cs2BiF7, containing about
50% CsF and a trace of CsBiF6.
energy with respect to Cartesian coordinates) were converted to
symmetry-adapted internal coordinates for further analysis using the
program systems GAMESS and Bmtrx.30
Figure 4. Vibrational spectra of Cs2SbF7. (A) Infrared spectrum of
an AgBr disk recorded at room temperature. (B and C) Raman spectra
Results and Discussion
recorded at -130 and 25 °C, respectively. The bands marked by an
2-
Syntheses and Properties of BiF7 Salts. The syntheses
-
asterisk are attributed to a small amount of SbF6
.
2-
of the Na+, K+, Rb+, and Cs+ salts of BiF7 are surprisingly
simple and were achieved by heating BiF5 with an excess of
the corresponding alkali metal fluoride to temperatures above
230 °C for several days in a Monel reactor (eq 1).
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision E.2; Gaussian,
Inc.: Pittsburgh, PA, 1995.
2MF + BiF5 230-310 °C8
M2BiF7
(1)
(M ) Na, K, Rb, Cs)
To preempt a possible reduction of Bi(+V) to Bi(+III), a low
pressure of gaseous F2 was used as a protective gas during the
(30) Komornicki, A. Bmtrx, Version 2.0; Polyatomics Research Insti-
tute: Redwood City, CA, 1996.

2-
Table 1. Observed Vibrational Spectra of the BiF7 Dianion
obsd freq, cm-1 (rel intens)
Rb2BiF7
c
Na2BiF7
K2BiF7
Cs2BiF7
[N(CH3)4]2BiF7 (NO)2Bi7
approximate
Ra
Ra
Ra
Ra
IR-Ra
assignment in
mode
IR,
IR,
25 °C
IR,
25 °C
IR,
25 °C
Ra,
25 °C
IR,
Ra,
activity point group D5h
description
-130 °C
25 °C
25 °C -130 °C
25 °C
-130 °C
25 °C
-130 °C
25 °C
25 °C -130 °C
-Ra
A1′
ν1
ν2
νsym BiF5
νsym BiF2
561(100) 561(100)
553(100) 549(100)
548(100) 543(100)
540(100) 536(100)
538(100)
546(100)
549(47)
526(3)
519(4)
551(40)
520(2)
516(1)
520sh
530(6)
511(6)
510sh
519(1)
509(1)
520sh
505(2)
530sh
509(5)
IR-
IR-
A2′′
E1′
ν3
ν4
ν5
ν6
νasym BiF2
δumbrella BiF5
νasym BiF5
δasym BiF5
in-plane
(515vs,br)a
(287vs,br)b
515vs,br
534vs
528vs, br
280vs
525vs, br
508s
ν7
ν8
δsciss BiF2
287vs,br
-Ra
-Ra
E1′′
E2′
δwag BiF2
299(1)
278(0.5)
251(2)
233(0.5) 235sh
495(0.5)
299(1)
254(2)
284sh
267(5)
255(4)
240sh
279(3)
259(5)
249(6)
229(2)
299(0.5) 295(0+)
295(12)
277(6)
273sh
249(11)
276(1)
250(6)
270(1)
247(6)
238sh
252(6)
ν9
δasym BiF5
in-plane
490(1)
478(0.3) 466(2)
465(2)
376(3)
354(0.5) 358sh
490(2)
469(5)
465vw 444(3)
460(4)
438(2)
463w
459(5)
430(2)
413(2)
350(3)
331(6)
452(4)
427(2)
458vw
454(3)
431(0.5) 419(1)
424(1)
331(4)
450(3)
465(2)
453(1)
436(1)
353(4)
329(2)
409(1)
326(4)
ν10
ν11
νasym BiF5
375(4)
340(5)
332(5)
350sh
330(7)
-Ra
E2′′
δpucker BiF5
a The ν3 band is probably hidden underneath the very strong and broad ν5 band. b The ν4 band is probably hidden underneath the very stong and broad ν6 band. c In addition to the listed BiF72- vibrations,
the following bands were observed: 2280(9) and 2260(5), νNO+; 172(37,br), 125(3), 106(2), 99(9), 73(3), and 58(3), which are tentatively assigned to cation-anion interactions and/or lattice modes.

8396 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Drake et al.
2-
Table 2. Observed and Calculated Vibrational Spectra of the SbF7 Dianion
obsd freq, cm-1 (rel intens)
K2SbF7
Cs2SbF7
unscaled calcd freq
cm-1 (IR intens)a
Ra
Ra
assignment in
point group D5h
-130 °C
25 °C
IR, 25 °C
-130 °C
25 °C
IR, 25 °C
HF/1 HF/5
A1′
A2′′
E1′
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
605(100)
520(7)
604(100)
517(6)
596(100)
512(1)
597(100)
520sh
608(0)
527(0)
640(193)
345(107)
559(346)
392(366)
232(7)
293(0)
501(0)
377(0)
34(0)
602(0)
522(0)
635(199)
342(107)
551(185)
384(201)
233(2)
292(0)
491(0)
369(0)
64(0)
627sh
585vs,br
342vs,br
574vs
335vs
E1′′
E2′
294(17)
495(6)
396(7)
297(20)
495(4)
395(5)
289(6)
490(1.5)
392(5)
285(5)
492(2)br
391(5)
E2′′
a For basis sets, see text.
Table 3. Comparison of Experimental Frequencies (cm-1) of
Cs+ salts, and ClF,35 ClF3,36 ClF3O,37 and IF538 form only stable
K+, Rb+, and Cs+ salts.
2-
2-
BiF7 and the Isoelectronic Series, IF7, TeF7-, SbF7
a
- b
2- c
2- c
IF7
TeF7
SbF7
BiF7
By analogy to the halogen (see above) or xenon fluorides or
oxofluorides,31-38 BiF5 also forms at -78 °C a thermally
unstable (NO)2BiF7 salt (eq 2a) which, on warming to room
temperature, loses 1 mol of FNO to give stable NO+BiF6- (eq
2b).
A1′
A2′′
E1′
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
676
635
746
365
670
425
257
319
596
510
640
597
699
335
625
385
596
512
545
510
627
(300)d
574
520
284
335
2FNO + BiF5 -78 °C 8 (NO)2BiF7
(2a)
(2b)
FNO(liq)
E1′′
E2′
295
(540)d
458
289
490
392
260
460
330
(NO)2BiF7 25 °C8 NOBiF6 + FNO
vac
E2′′
a Data from ref 39. b Data from ref 40. c Data from this study.
d Predicted values, based on the trends observed in this table.
A stable [N(CH3)4]2BiF7 salt was also prepared. Due to the
limited thermal stability and oxidizer resistance of N(CH3)4F,15
a thermal synthesis at 250 °C was not feasible, and a low-
temperature synthesis in CH3CN solution had to be employed
(eq 3).
heating. However, excessive heating and significant fluorine
pressures must be avoided, as they can result in attack of the
2-
Monel cylinder with the formation of some NiF6 impurities,
which can be easily detected by their pink color and intense
Raman bands at about 560, 519, and 294 cm-1. To suppress
2N(CH3)4F + BiF5 -31 °C8 [N(CH3)4]2BiF7
(3)
-
the simultaneous formation of some BiF6 salts, it is advanta-
CH3CN
geous to use a 50% molar excess of the alkali metal fluorides.
Even with a large excess of alkali metal fluoride, there was no
Removal of the CH3CN solvent at -22 °C resulted in a product
3-
evidence for the formation of BiF8 salts under these condi-
which contained mainly [N(CH3)4+]2BiF7 but also some smaller
tions. Attempts to prepare Li2BiF7 under similar conditions
were also unsuccessful. The lack of Li2BiF7 formation is not
surprising in view of the previous findings that the tendency of
alkali metal fluorides to form stable complex fluoroanion salts
with covalent main group fluorides or oxofluorides decreases
from Cs+ to Li+. The main driving forces for salt formation
are the fluoride ion affinities of the covalent main group
fluorides and the lattice energy differences between the fluorides
and the complex fluoroanion salts of the alkali metals. Without
exact knowledge of these values, it is difficult to predict exactly
where the cutoff limit for salt formation lies for a given system
and can vary somewhat from one system to another. Thus,
SbF5,13 BiF5,14 and IF3O 31 can form stable Li+, Na+, K+, Rb+
and Cs+ salts of SbF6-, BiF6-,and IF5O-, respectively, while
BiF3,32 SeF4,33 and SeF634 form only stable Na+, K+, Rb+, and
+
amounts of N(CH3)4 salts of BiF6-, HF2-, and H2F3-, with
the bifluoride anions being generated by hydrogen abstraction
from CH3CN by either F- 19 or the bismuth fluorides.
The alkali metal BiF72- salts are thermally stable, crystalline,
white solids which exhibit little solubility in convential organic
solvents. Attempts to dissolve the compounds in anhydrous
HF, resulted in the following displacement reaction (eq 4).
HF
M2BiF7 + HF 25 °C8 MHF2 + MBiF6
(4)
Similarly, when N(CH3)4F and SbF5 in a 2.2:1 mole ratio were
combined at -45 °C in SO2 solution, only N(CH3)4+SbF6- and
N(CH3)4+SO2F- were obtained in quantitative yield. This
indicates that, in SO2 solution, these heptafluoropnictate(V)
dianions also undergo solvolysis (eq 5).
(31) Christe, K. O.; Wilson, W. W.; Wilson, R. D. Inorg. Chem. 1989,
28, 904.
(35) Christe, K. O.; Guertin, J. P. Inorg. Chem. 1965, 4, 1785.
(36) (a) Christe, K. O.; Wilson, W. W.; Wilson, R. D. Inorg Chem. 1989,
28, 675. (b) Wilson, W. W.; Christe, K. O. Inorg. Chem. 1989, 28, 4172
and references therein.
(37) Christe, K. O.; Schack, C. J.; Pilipovich, D. Inorg. Chem. 1972,
11, 2205.
(32) (a) Stein, L. In Halogen Chemistry; Gutmann, V., Ed.; Academic
Press: New York, 1967; Vol. 1, p 133. (b) Rhein, R. A.; Miles, M. H.
Technical Report 6811; Naval Weapons Center: China Lake, CA, 1988.
(33) Christe, K. O.; Curtis, E. C.; Dixon, D. A.; Mercier, H. P.; Sanders,
J. C. P. Schrobilgen, G. J. J. Am. Chem. Soc. 1991, 113, 3351.
(34) Christe, K. O.; Wilson, W. W. Inorg. Chem. 1982, 21, 4113.
(38) Christe, K. O. Inorg. Chem. 1972, 11, 1215.

SeVen-Coordinated Pnicogens
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8397
Table 4. Unscaled Calculateda Vibrational Frequencies (cm-1) and IR Intensities (km/mol) of D5h BiF7
2-
assignment
HF/1
HF/2
MP2/2
HF/3
MP2/3
HF/4
MP2/4
HF/5
MP2/5
A1′
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
568(0)
502(0)
581(0)
521(0)
511(0)
489(0)
532(125)
271(84)
501(226)
314(223)
187(10)
242(0)
434(0)
381(0)
18i
579(0)
495(0)
482(0)
445(0)
495(181)
254(98)
448(325)
291(322)
179(9)
230(0)
397(0)
331(0)
9i
572(0)
505(0)
483(0)
458(0)
502(153)
261(80)
465(276)
295(254)
185(6)
233(0)
403(0)
348(0)
6i
565(0)
490(0)
474(0)
447(0)
490(157)
256(76)
454(136)
291(125)
180(3)
231(0)
396(0)
341(0)
32(0)
A2′′
E1′
576(166)
287(106)
527(298)
337(300)
197(15)
255(0)
456(0)
387(0)
46i
582(144)
295(115)
547(279)
340(298)
203(15)
263(0)
462(0)
404(0)
24i
578(173)
289(106)
510(301)
330(351)
204(13)
261(0)
456(0)
352(0)
17i
573(164)
292(115)
525(313)
329(340)
206(10)
262(0)
448(0)
375(0)
25(0)
561(167)
289(113)
516(308)
327(342)
203(10)
260(0)
442(0)
364(0)
32(0)
E1′′
E2′
E2′′
a For a description of the basis sets used, see text.
SO2
of (NO)2SbF7 in the SbF5/FNO system at -78 °C, with
SbF72- + SO2
8 SbF6- + SO2F-
(5)
-
NO+SbF6 being the only observable product.
Attempted Synthesis of AsF72- Salts. Attempts to prepare
Cs2AsF7 by heating AsF5 with a 3-fold excess of CsF for 4
days to 230 °C produced exclusively CsAsF6. Similarly,
reactions of AsF5 with excess N(CH3)4F in CH3CN at -31 °C
gave no evidence for the formation of [N(CH3)4]2AsF7.
Although the ab initio calculations (see below) predict that
2-
- 14
The hydrolysis of BiF7 salts is analogous to that of BiF6
;
i.e., the white solids first turn yellow when exposed to moist
air and, when added to water, first form yellow solutions which,
after several minutes, turn dark brown due to the formation of
high-oxidation-state bismuth oxides. The only difference from
BiF6- salts is that, while for BiF6- these reactions are practically
instantaneous, for BiF72- they take several minutes to proceed;
i.e., the increased coordination sphere around the central atom
and the double negative charge of the dianions apparently retard
the attack by OH- ions.
2-
the free gaseous AsF7 ion is vibrationally stable toward F-
2-
loss, the formation of stable solid AsF7 salts appears to be
energetically unfavorable relative to that of MAsF6 and MF.
Thus, the propensity of pnicogen pentafluorides to form
heptafluoro dianions decreases in the order BiF5 > SbF5 > AsF5,
as expected from the decreasing size of the central atom and
the decreasing F- ion affinity of the pentafluorides.
The M2BiF7 salts were shown by Raman spectroscopy (see
2-
below) to be distinct compounds containing the BiF7 anion
-
and not mixtures of BiF6 and F- ions. Based on their
preparative conditions, the Na+, K+, Rb+, and Cs+ salts of
BiF72- are thermally stable up to at least 300 °C. The thermal
stability of [N(CH3)4]2BiF7 is limited by the stability of the
organic cation, which, as in N(CH3)4F,19 is prone to lose CH3F
on pyrolysis.
Vibrational Spectra. In the absence of single-crystal X-ray
and NMR data, a combination of vibrational spectroscopy and
ab initio calculations was used to characterize these novel
heptacoordinated dianions. The observed Raman and infrared
spectra are shown in Figures 1-4, and their frequencies and
assignments have been summarized in Tables 1 and 2. Based
on the results from the ab initio calculations (see below) and a
comparison with the known spectra of isoelectronic pentagonal
2-
Syntheses and Properties of SbF7 Salts. Antimony
pentafluoride is also capable of forming a stable dianion with
excess alkali metal fluorides. However, contrary to BiF5, SbF5
does not form a stable sodium salt, and only salts with potassium
or heavier alkali metals are obtainable. The salts were prepared
by heating either SbF5 with excess KF or CsF to 220-300 °C
(eq 6a) or, preferably, CsSbF6 with 2 mol of CsF to 300 °C (eq
6b).
39
- 40
bipyramidal IF7 and TeF7
(see Table 3), the observed
spectra are assigned in point group D5h and establish the
2-
2-
pentagonal bipyramidal structures of BiF7 and SbF7
.
2MF + SbF5 220-300 °C8 M2SbF7
(M ) K, Cs)
(6a)
CsSbF6 + CsF 300 °C8 Cs2SbF7
(6b)
Surprisingly, it was found that CsSbF6, when ball-milled for 1
h at room temperature in a high-frequency Wig-L-Bug, showed
a 13% conversion to Cs2SbF7, indicating that the activation
As can be seen from Figure 1 and Table 1, the spectra of
BiF7 are significantly influenced by the counterion. By
2-
2-
energy barrier toward SbF7 formation is low. The heat
analogy with previous findings for different alkali metal salts
of IF4O- 1 and BrF4O-,41 the band splittings increase with
decreasing cation size, and the frequencies vary slightly from
salt to salt, in accord with increasing anion-cation interactions.
Additional evidence for increased cation-anion interaction with
decreasing cation size comes from the Raman spectrum of
required for the thermal syntheses of these salts probably serves
only the purpose of increasing the ion mobilities and might
involve melting or volatilization of either the starting materials
or the reaction products.
2-
2-
The alkali metal SbF7 salts, like their BiF7 analogues,
are stable, white, crystallinic solids with little solubility in
organic solvents. In inorganic solvents, such as HF or SO2,
they readily undergo solvolysis accompanied by loss of one F-
ion (see above), thus foiling their study by NMR spectroscopy.
Antimony pentafluoride also forms a stable [N(CH3)4]2SbF7
2-
(NO)2BiF7 (see Table 1 and Figure 3). Whereas the BiF7
(39) Christe, K. O.; Curtis, E. C.; Dixon, D. A. J. Am. Chem. Soc. 1993,
115, 1520 and references therein.
(40) Christe, K. O.; Dixon, D. A.; Sanders, J. C. P.; Schrobilgen, G. J.;
Wilson, W. W. J. Am. Chem. Soc. 1993, 115, 9461.
(41) Christe, K. O.; Wilson, R. D.; Curtis, E. C.; Kuhlmann, W.;
Sawodny, W. Inorg. Chem. 1978, 17, 533.
2-
salt, which was prepared by analogy to the BiF7 salt at -31
°C in CH3CN solution. However, low-temperature Raman
(42) Marx, R.; Mahjoub, A. R.; Seppelt, K.; Ibberson, R. M. J. Chem.
Phys. 1994, 101, 585.
spectroscopy failed to provide any evidence for the formation

8398 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Drake et al.
frequencies are comparable with those in the alkali metal salts,
the NO+ stretching frequency (2280 and 2260 cm-1) has
decreased approximately 70 cm-1 from that (2338 cm-1) found
for the more ionic NO+BiF6 salt (see Experimental Section).
-
Furthermore, the Raman spectra exhibit a very pronounced
temperature dependence, as is expected for highly fluxional
pentagonal bipyramidal species.39,40 At room temperature, they
undergo a rapid dynamic puckering of the highly congested
equatorial plane and a slower intramolecular exchange of the
axial and equatorial fluorine ligands.39 This causes the room-
temperature Raman bands to become very broad and to lose
most of their fine structure.
Recording reliable infrared spectra in the low-frequency range
was somewhat problematic since an excess of alkali metal
fluoride, which also absorbs in this range, had to be used in the
-
syntheses to suppress the formation of the octahedral XF6
monoanions. With the heaviest alkali metal fluoride, i.e., CsF,
the 290-cm-1 IR band could still be well established (see Figure
2). However, for the lighter alkali metal fluorides, which start
to absorb already at higher frequencies, some interference with
2-
the XF72- deformation modes was encountered. For the SbF7
salts this was less of a problem than for BiF72-, because the ν6
2-
antisymmetric deformation mode of SbF7 occurs at a higher
frequency (∼335 cm-1). However, the similar frequencies of
ν3 and ν5 and ν4 and ν6 and the broadness of their infrared bands
rendered the observation of ν3 and ν4 somewhat uncertain. As
2-
can be seen from Figure 4, the ν5 infrared band of SbF7
exhibits in the region expected for ν3 a shoulder at 627 cm-1
and a weak band at 660 cm-1. Since the latter frequency
coincides with ν3 of SbF6- and the Raman spectrum established
-
the presence of a small amount of SbF6 in our sample, the
627-cm-1 shoulder is assigned to ν3 of SbF72-. Similarly, there
are several shoulders on the low-frequency side of the 335-
cm-1 ν6 band of SbF72-, and one of these could be due to ν4 of
2-
SbF7
.
Electronic Structure Calculations and Vibrational As-
signments. The well-known and closely related octahedral
BiF6-, SbF6-, and AsF6- anions were used to test the different
methods and basis sets used in our calculations. As can be seen
from Table 5, the Hartree-Fock calculations with basis Sets 1
and 5, denoted as HF/1 and HF/5, gave the best agreement with
the observed frequencies and geometries and were, therefore,
preferentially used for all the other calculations in this study.
In agreement with the known experimental D5h structures of
39,42
- 10,40
isoelectronic IF7
and TeF7
,
our ab initio calculations
for BiF72-, SbF72-, and AsF72- result in pentagonal bipyramids
of D5h symmetry (see Table 6) as the minimum energy
-
structures. Within the isoelectronic series XeF7+, IF7, TeF7
,
and SbF72-, the energy differences between the pentagonal
bipyramidal (PBP) structure and the monocapped trigonal
prismatic (MCTP) and monocapped octahedral (MCO) struc-
tures were calculated at the MP2 level to be about 3-4 kcal/
mol, with the differences between the MCTP and the MCO
structures being very small, favoring the MCTP by about 0.2
-
kcal/mol. Assuming similar bond length deviations as for BiF6
,
SbF6-, and AsF6- (see Table 5), the most likely geometries of
BiF72-, SbF72-, and AsF7 can be predicted and are sum-
2-
2-
marized in Table 6. The calculated frequencies of BiF7
,
SbF72-, and AsF7 are summarized in Tables 4 and 7, 2 and
2-
8, and 9, respectively. The imaginary frequencies found for
ν
11 of BiF72- in some of the calculations, given in Table 4, are
probably due to the numerical differentiation used in our
calculations and can be avoided by appropriate modification of
the basis set (see Table 4).

SeVen-Coordinated Pnicogens
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8399
Table 6. Calculated and Predicted Geometries for the D5h BiF72-, SbF72-, and AsF7 Dianionsa
2-
calcdb (predicted) bond lengths (Å)
2-
2-
2-
BiF7
SbF7
AsF7
HF/1
HF/2
HF/3
HF/4
HF/5
MP2/2
MP2/5
HF/1
HF/5
HF/1
HF/5
rX-Fax
rX-Feq
1.951
2.008
1.958
2.009
1.945
2.009
1.957
2.016
1.945
2.009
2.010
2.055
2.024 (1.96)
2.081 (2.03)
1.875
1.953
1.878 (1.89)
1.959 (1.97)
1.718
1.847
1.723 (1.74)
1.861 (1.87)
a For D5h symmetry, all bond angles are the same:
FeqXFeq ) 72° and FaxXFeq ) 90°. b For basis sets, see text.
Table 7. Unscaled HF/DZP/ECP (HF/1) Symmetry Force
Constants and Potential Energy Distribution of D5h BiF7
of the observed frequency deviations. The extra splittings of
some of the bands observed, particularly for the low-temperature
spectra of BiF72-, can be explained by the lifting of the
degeneracies for the doubly degenerate E modes and either site
symmetry effects or in-phase/out-of-phase coupling of more than
one molecule per unit cell. In the infrared spectra of some of
2-
calcd freq,
cm-1
symmetry force constantsa
PEDb
F11
F11 3.25
F22 0.392
F33
F22
A1′ ν1
568
502
55(1) + 45(2)d
55(2) + 45(1)e
ν2
3.17
F44
2-
the BiF7 salts, generally a very weak but sharp band was
observed at about 460 cm-1 which had the same frequency as
one of the degenerate components of the Raman active ν9 (E2′)
mode and, therefore, is tentatively assigned to this mode. Also,
in the higher frequency region of the infrared spectra, frequently
some weak bands at 1050 and 960 cm-1 could be observed
which can be attributed to the combination bands (ν1 + ν5) (E1′)
and (ν5 + ν9) (E1′ + E2′), respectively. Overall, the very good
agreement between the calculated and observed spectra leaves
no doubt that BiF72- and SbF72- possess pentagonal bipyramidal
D5h structures.
A2′′ ν3
576
287
F33 3.17
F44 0.196
F55
F55 2.65
F66 0.893
F77 0.169
F88 0.633
F99
97(3) + 3(4)
ν4
1.35
F66
100(4)
F77
E1′ ν5
ν6
ν7
E1′′ ν8
527
337
197
255
99(5)
61(6) + 30(7) + 8(5)
2.92
-0.359 0.716 92(7) + 7(6)
100(8)
F10,10
E2′ ν9
456
387
45ic
F99 2.40
F10,10 0.302
F11,11 (-0.048)c
71(9) + 29(10)
75(10) + 25(9)
ν10
1.85
E2′′ ν11
a Stretching force constants in mdyn/Å, deformation constants in
mdyn Å/rad2, and stretch-bend interaction constants in mdyn/rad.
b PED in percent. The symmetry coordinates are defined as (1) νsym
BiF5 eq; (2) νsym BiF2ax; (3) νasym BiF2; (4) νumbrella BiF5; (5)
νasym BiF5; (6) νasym BiF5 in-plane; (7) δsciss BiF2; (8) δwag BiF2;
(9) δasym BiF5 in-plane; (10) νasym BiF5; (11) δpucker BiF5. c The
negative value for F11,11 is due to the imaginary frequency which was
caused by the use of numerical differentiation in the calculation.
d Symmetric combination of S1 and S2. e Antisymmetric combination
of S1 and S2.
Additional support for the pentagonal bipyramidal structure
2-
of SbF7 comes from the smooth frequency trends observed
2-
for the isoelectronic IF7, TeF7-, SbF7 series (see Table 3).
The frequency and, particularly, the relative Raman intensity
trends observed for the symmetric equatorial and the symmetric
axial stretching modes ν1 and ν2, respectively, which have
previously been discussed already in detail for TeF7 ,
- 40 are even
more pronounced for SbF72- and BiF72-. Thus, the mixing of
the symmetry coordinates of ν1 and ν2 increases further when
Table 8. Unscaled HF/DZP/ECP (HF/1) Symmetry Force
Constants and Potential Energy Distribution of D5h SbF7
going from TeF7 to SbF7 and BiF72-, as shown by the
potential energy distributions (PED) given in Tables 7 and 8.
The ν1 and ν2 modes of SbF72- and BiF72- have become almost
equal mixes of the corresponding symmetry coordinates, with
ν1 being their symmetric and ν2 their antisymmetric combina-
tion. This accounts for the unusually high relative Raman
-
2-
2-
calcd freq,
cm-1
symmetry force constantsa
PEDa
F11
F11 3.60
F22 0.514 3.65
F22
A1′ ν1
608
527
47(1) + 53(2)b
53(1) + 47(2)c
ν2
2-
F33
F44
intensity of ν1 and the low intensity of ν2 in both SbF7 and
A2′′ ν3
640
345
F33 3.58
F44 0.394 1.59
93(3) + 7(4)
100(4)
2-
BiF7 and eventually even leads to an identity reversal of ν1
ν4
2-
2-
and ν2 on going from BiF7 to AsF7 (see PED of Table 9).
A similar identity reversal is also exhibited for ν9 and ν10 in
F55
F66
F77
E1′ ν5
ν6
559
392
232
293
F55 2.79
F66 1.17 3.33
F77 0.298 -0.388 0.902 90(7) + 10(6)
F88 0.779
F99
97(5)
53(6) + 37(7) + 10(5)
the E2′ block. While in IF7 the higher frequency mode is mainly
ν7
2-
the antisymmetric, in-plane IF5 stretching, in BiF72-, SbF7
,
E1′′ ν8
100(8)
and AsF72-, the higher E2′ frequency mode has increasingly
become the antisymmetric, in-plane deformation (see Tables
7-9).
F10,10
E2′ ν9
501
377
34
F99 2.79
F10,10 0.419 1.75
F11,11 0.025
87(9) + 13(10)
88(10) + 12(9)
100(11)
ν10
E2′′ ν11
2-
The general frequency decrease on going from IF7 to SbF7
a For dimensions and the definitions of the symmetry coordinates,
see footnotes of Table 7. b Symmetric combination of S1 and S2.
c Antisymmetric combination of S1 and S2.
in Table 3 agrees well with our expectations. With increasing
formal negative charges of the ions, the (δ+)X-F(δ-) polarity
of the X-F bonds increases, which increases their ionicity and
weakens them. A similar bond-weakening effect is also
observed on going from SbF72- to BiF72- (see Tables 3). That
Inspection of Tables 1, 2, and 4 reveals that, for BiF72- and
SbF72-, the unscaled calculated HF/1 and HF/5 frequencies
agree reasonably well with the experimental ones and, therefore,
require no scaling. Some of the deviations between observed
2-
2-
the frequency lowerings from SbF7 to BiF7 cannot exclu-
sively be a mass effect is evident from the decrease in the
frequencies of ν1, in which the central atom does not move.
2-
and calculated frequencies, such as for ν5 and ν6 of SbF7 in
The symmetry force constants and potential energy distribu-
Table 2, could be due to the presence of two strongly coupled
vibrations within the same symmetry species, as shown by their
relatively large interaction force constants (F56 in Table 8). Small
variations in these interaction constants can strongly affect the
frequency separation between these modes and account for some
tions for BiF72-, SbF72-, and AsF72- are summarized in Tables
2-
7-9. Since for some of the infrared active modes of BiF7
2-
2-
and SbF7 and for all modes of AsF7 no experimental data
were available, the unscaled HF/DZP/ECP (HF/1) frequencies

8400 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Drake et al.
2-
(fdd′) remains negligible. Within the BiF72-, SbF72-, and AsF7
Table 9. Unscaled HF/DZP/ECP (HF/1) Frequencies Symmetry
Force Constants and Potential Energy Distribution of D5h AsF7
2-
series, where the number of formal negative charges is constant
but the size and the mass of the central atom decrease, very
interestingly the force constant of the more covalent axial bonds
calcd freq,
cm-1
symmetry force constantsa
PEDa
increases from BiF72- to AsF72- while at the same time that of
F11
F11
F22
F22
3.89
0.693 4.31
F33
A1′ ν1
656
549
65(2) +35(1)b
65(1) +35(2)c
2-
the more ionic equatorial bonds has its lowest value for AsF7
.
ν2
This effect might be explained by the increasing tendency of
F44
2-
the XF7 species to lose with decreasing size of the central
A2′′ ν3
742
430
F33
F44
4.23
0.591 1.93
F55
83(3) +17(4)
100(4)
-
atom one fluoride ion with formation of a more covalent XF6
ν4
anion. Similar to the XeF7+, IF7, TeF7-, and SbF7 series,
2-
F66
F77
E1′ ν5
ν6
567
470
294
357
F55
F66
F77
F88
2.69
1.75 4.21
0.477 -0.387 1.18 85(7) +13(6) +2(5)
0.994
F99
98(5) +2(7)
there is again a very strong increase in fdd, the coupling constant
of the adjacent equatorial fluorine ligands, despite a decrease
in the fd values.
37(7) +31(5) +31(6)
ν7
E1′′ ν8
100(8)
F10,10
Although explicit values for the internal deformation constants
cannot be given without the assumption of certain constraints,
several general conclusions concerning the relative magnitudes
of the in-plane (fR) and out-of-plane (fâ) deformation constants
and their trends can be reached from an inspection of their
corresponding symmetry force constants in Tables 7-9 and ref
39 and 40 (it should be noted that Table 1 of ref 40 contains an
error in the approximate mode descriptions for ν6 and ν7, which
have been transposed). As expected, the equatorial in-plane
deformation constants, fR, increase with increasing stiffening
of the X-F bonds on going from SbF72- to XeF7+, which to a
lesser extent is also the case for the out-of-plane deformation
constant, fâ. Similarly, the increases in both fR and fâ on going
from BiF7 to AsF7 are expected on the basis of the
shortening of the bonds. This strong increase in the deformation
force constants, coupled with a simultaneous slight decrease of
the equatorial stretching force constant, accounts for the
increased coupling between stretching and deformation modes
and some of the observed identity reversals encountered on
E2′ ν9
600
291
91
F99
3.67
96(9) +4(10)
95(10) +5(9)
100(11)
ν10
F10,10 0.610 1.08
F11,11 0.159
E2′′ ν11
a For dimensions and definitions of the symmetry coordinates, see
footnotes of Table 7. b Symmetric combination of S1 and S2. c Anti-
symmetric combination of S1 and S2.
Table 10. Internal Stretching Force Constantsa (mdyn/Å) of the
2-
Isoelectronic Series XeF7+, IF7, TeF7-, and SbF7 and of the
Pnicogen(V) Heptafluoride Dianion Series (b)
series a
series b
+
-
2-
2-
2-
2-
XeF7
IF7
TeF7
SbF7
BiF7
SbF7
AsF7
fD(ax)
fDD
fd(eq)
5.046 5.005 4.415 3.61
0.088 0.058 0.045 0.04
4.144 3.947 3.091 2.534 2.434 2.534 2.286
3.21
0.04
3.61
0.04
4.27
0.04
2-
2-
fdd(adjac) 0.117 0.326 0.538 0.500 0.363 0.500 0.761
fdd′(oppos) 0.034 0.047 -0.003 0.033 0.005 0.033 0.041
a It should be noted that the values for XeF7+, IF7, and TeF7- from
refs 39 and 40 are scaled by factors of 0.8636 and 0.8649, respectively,
2-
2-
2-
2-
whereas the SbF7 and AsF7 values are unscaled values from this
going from BiF7 to AsF7
.
2-
study. The SbF7 values were not scaled in view of the very close
Conclusions
agreement between experimentally observed and calculated frequencies
(see Table 3), which eliminated the need for using a scaling factor.
In view of the long known existence and widespread use of
the pnicogen(V) hexafluoride anions and the easy syntheses and
great stabilities of the previously unknown pnicogen(V) hep-
tafluoride dianions, it is truly amazing that no previous reports
on the existence of these dianions could be found in the
literature. This appears to be another example for general
preconceptions about what may and may not exist, having
stopped chemists from openmindedly searching for certain
simple species. It is this kind of dogmatic thinking (in our case,
the magic stability attributed to the octahedral “sp3d2” type
electronic configurations) which had delayed for a long time
the discovery of noble gas compounds.49
Acknowledgment. This article is dedicated to Prof. George
A. Olah on the occasion of his 70th birthday. The authors thank
Drs. S. Rodgers, P. Carick, and G. Olah for their active support.
The work at the Air Force Research Laboratory was financially
supported by the Propulsion Directorate and Office of Scientific
Research of the Air Force, that at USC by the National Science
Foundation, and that at PNNL by the Department of Energy.
G.W.D. gratefully acknowledges the National Research Council
for a postdoctoral fellowship at AFRL.
and force fields were used for all three anions to allow a better
comparison. Most of the implications of this normal coordinate
analysis have already been discussed and do not need to be
reiterated, except for emphasizing that, when comparing the
39
results from this study with the previous analyses of IF7 and
- 40
TeF7
,
attention must be paid to the different identities, i.e.,
reversal of assignments, for ν1 and ν2 and ν9 and ν10.
Of the internal force constants, only the stretching force
constants are fully determined39 and are given in Table 10. For
all compounds, the axial stretching force constants are signifi-
cantly larger than the equatorial ones. This finding is in
excellent agreement with the bonding scheme previously
proposed39 for IF7. It involves a planar, delocalized pxy hybrid
orbital of the central atom for the formation of five equatorial,
semi-ionic, 6-center, 10-electron bonds and an spz hybrid for
the formation of two mainly covalent axial bonds. Depending
on the degree of congestion in the equatorial plane, the equatorial
fluorine ligands can undergo varying degrees of fast dynamic
puckering which, together with a slower intramolecular equato-
rial-axial ligand exchange, account for the fluxionality of these
molecules.39 As expected, increasing formal negative charges
increase the (δ+)X-F(δ-) polarity and, hence, the ionic
character of the X-F bonds and explain the observed bond-
JA9805728
(43) Bougon, R.; Charpin, P.; Christe, K. O.; Isabey, J.; Lance, M.;
Nierlich, M.; Vigner, J.; Wilson, W. W. Inorg. Chem. 1988, 27, 1389.
(44) Christe, K. O.; Wilson, R. D.; Schack, C. J. Inorg. Chem. 1977,
16, 937.
+
weakening encountered on going from XeF7 to SbF72-. It is
interesting to note the strong influence the increasing formal
negative charges have on fdd, the coupling constant between
adjacent equatorial X-F bonds. With increasing ionicity of
the X-F bonds, the coupling between adjacent bonds strongly
increases, while the coupling between opposite equatorial bonds
(45) Siebert, H. Anwendungen der Schwingungsspektroskopie in der
Anorganischen Chemie; Springer-Verlag: Berlin, 1966.
(46) Hebecker, C. Z. Anorg. Allg. Chem. 1971, 384, 12.
-
(47) Zhang, X.; Christe, K. O. Unpublished data for ClF4+SbF6
.
(48) Gafner, G.; Kruger, G. J. Acta Crystallogr., Sect. B 1974, B30, 250.
(49) Bartlett, N. Proc. Chem. Soc. 1962, 218.