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Acta Chim. Slov. 1999, 46(2), pp. 213-228
OXIDATION OF FIRST ROW TRANSITION ELEMENTS WITH AsF5 IN AsF3 AT AMBIENT TEMPERATURE: SYNTHESES AND STRUCTURES OF M(AsF6)2·2AsF3, M = Fe, Co, Ni †
Karel Lutar, Horst Borrmann, Adolf Jesih and Boris Ženiva
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
(Received 14.4.1999)
Abstract
A novel synthetic approach via reactions between pure metals (Cr, Mn, Fe, Co, Ni, Cu) and AsF5 in AsF3 as a solvent has been investigated. AsF5 proved to oxidize these metals to their (+2) oxidation state. In the case of Cr, Mn or Cu the same compounds as those in the superacidic systems MF2-AsF5-aHF (anhydrous hydrogen fluoride) were obtained: CrFAsF6, Mn(AsF6)2, and CuFAsF6, respectively. In the case of Fe, Co and Ni a new type of fluoroarsenates has been isolated: M(AsF6)2=2AsF3. The latter compounds are isostructural and crystallize in the monoclinic space group P21/n (no. 14) with Z=2. Unit cell dimensions of Fe(AsF6)2=2 AsF3 are: a = 733.8(4) pm, b = 1303.0(8) pm, c = 735.4(5) pm, 3 = 91.43(7)° at 292 K, and for Co(AsF6)2=2AsF3: a = 725.9(4) pm, b = 1292.9(7) pm, c = 727.6(5) pm, 3 = 91.64(5)° at 288 K. The structure is characterized by layers formed through transcoordinating AsF6 units in the basal plane of the octahedron about the transition metal. Both apical positions of this octahedron are occupied by AsF3 molecules. A main feature of the layers are characteristic rings resulting from the apex-sharing connectivity of AsF6 and MF6 octahedra. As AsF3 molecules of neighbouring layers are localized above and below the center of a particular ring, a stacking sequence ABAB... results.
Introduction
The most common method for the synthesis of metal(II)hexafluoroarsenates are
reactions between MF2 and excess of AsF5 in aHF [1,2]. AsF5 proved to be a strong enough fluorinating agent to bring about the in-situ synthesis of some metal(II)fluorides via the reaction between the respective metal and AsF5 in aHF. As by-product AsF3 is produced by these reactions. In order to have as clean a system  as possible it was
Dedicated to the memory of Prof. Dr. Jože Šiftar
214
decided to investigate aforementioned reactions between first row transition metals and AsF5 in AsF3 as a solvent. In the past AsF3 has rarely been used as solvent in respective reactions, most characteristic examples are reactions of sulphur, selenium or tellurium with fluoro acids like AsF5 or SbF5 in AsF3 [3-7]. In the current paper first results on reactions between some metals of the first transition series and AsF5 in AsF3 at ambient temperature are described.
Experimental
1.  Apparatus, reagents and techniques. The reactions were carried out in PFA reaction vessels (16 mm i.d. x 19 mm o.d.) equipped with Teflon valves and Teflon coated stirring bars. The part of the vacuum line used for manipulation of volatile materials was made entirely from Teflon, FEP or PFA and equipped with a Monel Helicoid pressure gauge (0-1500 torr (± 0.3%), Bristol Babcock). Nonvolatile materials were handled in the argon atmosphere of a drybox with maximum content of water vapours of 0.1 ppm (MBraun). Metals in all cases were fine powders and were used as supplied: chromium (BDH), manganese (Hopkin and Williams), iron (99.5%, Fluka), cobalt (-325 mesh, Alfa-Ventron), nickel (1Fm, Alfa-Ventron), and copper (BDH). AsF5 was prepared by the reaction of As2O3 with elemental fluorine as described for the synthesis of PF5 [8]. AsF3 was prepared according to a modified literature procedure [9] by the reaction of As2O3 with aHF in a nickel reaction vessel at 120°C. The purity of AsF5 and AsF3 was checked by IR spectroscopy. FeF2 and CoF2 were prepared from respective chlorides in a flow of aHF at elevated temperatures [10]. SbF3 was prepared from Sb2O3 and aqueous HF [11]. The purity of FeF2, CoF2 and SbF3 was checked by chemical analysis and X-ray powder diffraction patterns. SO2 (Merck) was purified according to standard procedures. Fluorine (99.8%, Solvay) was used as supplied. Anhydrous HF (99.9%, Praxair) was treated with K2NiF6 for several days prior to use. Infrared spectra were taken in a 10 cm nickel cell with AgCl windows on an FTIR spectrometer (Perkin Elmer 1710). X-ray powder diffraction patterns were obtained with a General Electric camera with Straumanis loading using Cu-K" radiation, and where high resolution was required on an image plate based Guinier camera (Huber) applying Co-Kai radiation.
2.  Chemical analysis. After complete decomposition of the sample in an alkaline melt total fluoride ion content was determined with an ion selective electrode using ORION 960 Autochemistry System Analyser. The amount of free fluoride ions was measured
215
directly by selective electrode after hydrolysis of the sample in distilled water. Arsenic present as AsF6- species was determined gravimetrically as insoluble tetraphenylarsonium salt. Total amount of arsenic was determined by the ICP method. Transition metals were determined by complexometric titrations.
3. Reactions of Cr, Mn, Fe, Co, Ni and Cu with AsF5 in AsF3. In a typical experiment few mmols of the particular metal were loaded into a PFA reaction vessel within the drybox. Then about 3-5 ml of AsF3 and an excess of AsF5 were added by condensation at 77 K on the vacuum system. Starting reaction conditions are shown in Table 1. All reactions were carried out at ambient temperature. Reactions are exothermic and all metals slowly start to dissolve. Reactions were finished in about twelve hours with the products being not completely dissolved. The colour of chromium reaction product was white with greenish tint. In the case of manganese the dense solution for several hours showed a red colour which later disappeared. The colour of the isolated compound was a very faint beige. In the case of iron the colour of solution was dark while the precipitate was grey. In the case of cobalt both the colour of solution and precipitate were a dark pink, while for nickel both phases showed a yellow-green. The colour of the copper compound was white. Finally volatiles were pumped away at room temperature, the pumping in dynamic vacuum being stopped when the change of weight was less than 5-10 mg/mmol?h. The largest vapour pressure created by decomposition was observed for the iron compound. The weight of reaction products is depicted in Table 1. In the case of Cr, Mn and Cu the mass balance corresponded to CrFAsF6, Mn(AsF6)2 and CuFAsF6 compounds, and X-ray powder diffraction patterns showed only the lines of these compounds [2,12]. The mass balances of iron (white beige), cobalt (pink) and nickel (light yellow) products corresponded to composition M(AsF6)2@2AsF3. Data of chemical analyses for these compounds are shown in Table 2.
4. Reaction of Co(AsF6)2 with AsF3. Co(AsF6)2 was prepared by literature method [1] from CoF2 and excess AsF5 in aHF, and its purity was checked by X-ray powder diffraction pattern. Onto 1.3540 g (3.10 mmol) Co(AsF6)2 in a PFA reaction vessel excess of AsF3 (15 mmol) was condensed at 77 K. Reaction was carried out at ambient temperature for five hours before excess of AsF3 was pumped away till constant weight of the sample was reached (2.1040 g). Theoretical increase of the weight for the composition Co(AsF6)2·2AsF3 is 0.8179 g (obtained increase was 0.7500 g). X-ray powder diffraction pattern of the obtained reaction product showed only lines of Co(AsF6)2@2AsF3.
216
Table 1. Starting Conditions for Reactions between M (M = Cr, Mn, Fe, Co, Ni, Cu) and AsF5 in AsF3, and Weights of Products
M	M		AsF5	AsF3	Weight of product (g)					
	g	mmol	mmol	ml	MFAsF6		M(AsF6)2		M(AsF6)2s2AsF3	
					Calcd.	Found	Calcd.	Found	Calcd.	Found
Cr	0.2188	4.21	20	5	1.0937	1.1074				
Mn	0.1608	2.93	20	3			1.2667	1.2631		
Fe	0.1905	3.41	20	3					2.3790	2.3694
Co	0.2395	4.06	20	5					2.8468	2.8377
Ni	0.1410	2.40	20	3					1.6824	1.6801
Cu	0.2415	3.80	20	4	1.0315	1.1302				
Table 2. Chemical Analyses for M(AsF6)2@2AsF3 (M = Fe, Co, Ni)
	Calcd. (%)					Found (%)				
	M	AsF6	Ft	Ff	As	M	AsF6	Ft	Ff	As
Fe(AsF6)2=2AsF3	8.00	54.17	49.03	16.34	42.96	8.8	57.0	48.8	15.0	39.9
Co(AsF6)2=2AsF3	8.41	53.93	48.81	16.27	42.78	8.4	53.9	48.5	16.3	42.7
Ni(AsF6)2=2AsF3	8.38	53.95	48.83	16.28	42.79	8.7	54.3	48.7	15.5	43.1
M = Fe, Co, Ni; Ft = total fluoride ion content; Ff = free fluoride ion content
5. Substitution reactions of M(AsF6)2·2AsF3 (M = Co, Ni) with SbF3 and SO2. Onto the mixture of Co(AsF6)2·2AsF3 (1.2069 g, 1.72 mmol) and SbF3 (0.6113 g, 3.42 mmol) about 3 ml of AsF3 were condensed at 77 K. The reaction was carried out at room temperature for one day. Then volatiles were pumped out and the weight of the obtained solid compound was 1.4324 g (calcd. weight for Co(AsF6)2·2SbF3 is 1.3682 g). Chemical analysis showed: calcd. for Co(AsF6)2·2SbF3: Co, 7.42; AsF6, 47.57; Sb, 30.66; Ff, 14.35%; found: Co, 7.3; AsF6, 43.0; Sb, 32.5; Ff, 16.4%. The weight of the obtained product from the analogous reaction between Ni(AsF6)2·2AsF3 (1.9451 g, 2.78 mmol) and SbF3 (1.0100 g, 5.65 mmol) in AsF3 was 2.4388 g (calcd. weight for Ni(AsF6)2·2SbF3 is 2.2278 g). Chemical analysis of this product showed: calcd. for Ni(AsF6)2·2SbF3: Ni, 7.39; AsF6, 47.58; Sb, 30.66; Ff, 14.36%; found: Ni, 7.1; AsF6, 45.6; Sb, 30.0; Ff, 15.5%.
In respective reactions SO2 acted both as reagent and as solvent. Onto 0.5417 g of Co(AsF6)2·2AsF3 (0.77 mmol) about 5 ml of SO2 were condensed at 77 K. At room temperature a red solution was obtained and reaction mixture was left for one day. Volatiles were separated by fractional condensation and eliminated AsF3 was identified
217
by IR spectroscopy. The weight of the obtained solid compound was 0.4650 g (calcd. weight for Co(AsF6)2·2SO2 is 0.4350 g). The reaction between Ni(AsF6)2·2AsF3 (0.7663g, 1.09 mmol) and about 5 ml of SO2 gave green-yellow solution. Again the release of AsF3 was shown by IR spectra of volatiles. The weight of obtained solid compound was 0.6528 g (calcd. weight for Ni(AsF6)2·2SO2 is 0.6145 g). The IR spectra of M(AsF6)2·2SO2 (M = Co, Ni) confirmed the presence of SO2 [13]. The colours of compounds containing SbF3 and SO2 are within slight variation of intensities the same as of respective AsF3 compounds.
6. Preparation of single crystals of M(AsF6)2·2AsF3 (M = Co, Fe). Several attempts were made to prepare single crystals of M(AsF6)2·2AsF3 (M = Fe, Co, Ni) compounds using supersaturated solutions in AsF3, unfortunately without success. Solutions of these compounds in aHF were also not suitable for growing crystals as they are quickly solvolysed loosing AsF3. Single crystals of Co(AsF6)2·2AsF3 were finally obtained from reaction between CoF3 (0.96 g, 8.3 mmol), AsF5 (7.68 g, 452 mmol), and aHF (0.35 g, 18 mmol) in a special stainless autoclave with a Teflon liner (V = 9.2 ml). The autoclave was heated for 18 days to 393 K, a temperature above the critical temperature of AsF5 [14]. A homogeneous batch of pink crystals was obtained. Colourless single crystals of Fe(AsF6)2·2AsF3 were also prepared in a solvothermal reaction. FeF2 (0.46 g, 4.90 mmol), AsF5 (3.80 g, 224 mmol) and AsF3 (2.11 g, 160 mmol) were heated in a stainless steel autoclave with a Teflon liner (V = 6.0 ml) at 383 K for five days. After isolation of the solid product some powder material was found, but majority was in the shape of well developed crystals.
7. Structural investigations. Crystals of Co(AsF6)2·2AsF3 and Fe(AsF6)2·2AsF3 are very sensitive even to traces of moisture. Therefore, they were selected in a drybox using a surgical microscope (f = 300 mm, Opton) and mounted into 0.3 mm quartz glass capillaries which were temporarily closed by Kel-F grease and later flame-sealed outside the drybox. Single crystal data for both compounds were collected on a diffractometer with an area detector (Stoe IPDS). Details of the diffraction experiments are given in Table 3. For crystallographic calculations the program systems SHELXTL-Plus [15] and SHELXL-97 [16] were used.
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Table 3. Crystal Data and Structure Refinement for Fe(AsF6)2·2AsF3
and Co(AsF6)2·2AsF3.
empirical formula formula weight crystal system space group
a, pm
b, pm
c, pm
ß, deg.
V, pm3
Z
T, K
d calc, g/cm3
A, pm
ju, mm-1
26>-range, deg.
reflections measured
independent refl., R int.
refinement method
extinction coeff.
no. parameters
R indices (all data)
Goof
Fe(AsF6)2·2AsF3		Co(AsF6)2·2AsF3
697.53		700.61
	monoclinic	
	P2 1/n (no.	14)
733.8(4)		725.9(4)
1303.0(8)		1292.9(7)
735.4(5)		727.6(5)
91.43(7)		91.64(5)
702.9(7) · 106	2	682.6(7) · 106
292 ± 2		288 ± 2
3.296	71.073	3.409
10.612		11.081
6 - 51		8 - 55
3936		3671
1183, 0.0326		1410, 0.0351
full matrix least-squares on F2
0.002(2) 109                                      110
R1 = 0.0364, wR2 = 0.0654   R1 = 0.0432, wR2 1.091                                    1.076
0.1023
8. Description of the M(AsF6)2*2AsF3 (M = Co, Fe) structures. The compounds Co(AsF6)2·2AsF3 and Fe(AsF6)2·2AsF3 turned out to be isostructural. They crystallize in the monoclinic space group P21/n (no. 14) with two formula units in the unit cell. Crystallographic data are given in Table 3, positional and isotropic displacement parameters for both compounds are given in Table 4, while interatomic distances and selected angles are compared in Table 5. Complete listings including anisotropic displacement parameters are available in supplementary material.
219
Table 4. Final Positional and Equivalent Displacement Parameters for Fe(AsF6)2-2AsF3 and CofAsFß)?2ASF3.
Atom
x
y
2a
Ueq(pm )
Fel	0	0	0	200(2)
Col	0	0	0	219(2)
Asl	0	0	0.5	267(2)
	0	0	0.5	271(2)
Fl	0.0507(5)	0.0276(2)	0.2707(3)	449(7)
	0.0543(5)	0.0268(3)	0.2696(4)	411(8)
F2	0.0679(7)	0.1179(3)	0.5601(4)	826(14)
	0.0698(7)	0.1190(4)	0.5614(5)	648(12)
F3	0.2129(6)	-0.0432(4)	0.5373(4)	832(14)
	0.2157(5)	-0.0437(4)	0.5400(5)	649(13)
As2	0.5	0	0	251(2)
	0.5	0	0	257(2)
F4	0.2699(4)	0.0311(2)	-0.0505(4)	440(7)
	0.2677(4)	0.0319(3)	-0.0510(4)	404(8)
F5	0.4412(5)	-0.0498(3)	0.2033(4)	604(10)
	0.4408(4)	-0.0497(4)	0.2062(5)	560(11)
F6	0.5291(5)	0.1153(3)	0.0967(5)	643(10)
	0.5314(5)	0.1168(3)	0.0974(6)	592(10)
As3	-0.01517(9)	0.28179(4)	0.06135(8)	423(2)
	-0.01707(8)	0.28127(5)	0.05960(9)	406(2)
F7	-0.0475(5)	0.1568(2)	-0.0368(4)	450(7)
	-0.0518(5)	0.1548(3)	-0.0390(4)	423(7)
F8	-0.1522(6)	0.2544(2)	0.2368(5)	589(9)
	-0.1516(6)	0.2538(3)	0.2391(6)	549(10)
F9	0.1792(6)	0.2449(3)	0.1716(6)	666(10)
	0.1813(6)	0.2443(4)	0.1672(7)	615(11)
z
) Ueq is defined as 1/3 of the trace of the orthogonalized Uij tensor.
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Table 5. Selected Interatomic Distances (pm) and Angles (deg.) for Fe(AsF6)2·2AsF3
and for Co(AsF6)2·2AsF3.
M = Fe
M = Co
M - F1		204.8(3)	2x	202.0(3) 2x
- F4		206.4(3)	2x	203.1(3) 2x
- F7		208.9(3)	2x	205.4(4) 2x
As1 - F2		167.1(4)	2x	167.7(4) 2x
- F3		167.7(4)	2x	168.2(4) 2x
- F1		177.2(3)	2x	176.7(3) 2x
As2 - F6		167.4(3)	2x	168.1(4) 2x
- F5		169.5(3)	2x	169.8(3) 2x
- F4		176.7(3)	2x	176.5(3) 2x
As3 - F9		169.3(4)		168.8(4)
- F8		169.3(4)		169.1(4)
- F7		179.5(3)		180.1(3)
F1 - M -	F4	89.4(1) 2x		89.0(1) 2x
F1 - M -	F4’	90.6(1) 2x		91.0(1) 2x
F1 - M -	F7	88.8(1) 2x		89.8(1) 2x
F4 - M -	F7	86.7(1) 2x		87.1(1) 2x
As1 - F1	- M	148.4(2)		147.7(2)
As2 - F4	- M	146.4(2)		145.8(2)
As3 - F7	- M	144.6(2)		143.8(2)
As requested from the number of formula units the transition metal has to be located on a center of symmetry. The basic structural unit is depicted in Figure 1. Atoms As1 and As2 also occupy centers of symmetry resulting in an arrangement with rather high symmetry. Both AsF6 octahedra about these arsenic atoms are bridging transition metal atoms through trans fluorine ligands. An immediate consequence of this particular arrangement is the formation of a layer composed from a square network. The interatomic distances between the transition metal center and its fluorine ligands within the layer are not significantly different within the error limits, but clearly all Fe-F distances are slightly longer than respective Co-F distances. The same holds for the
221
apical fluorine ligands which are provided by AsF3 molecules. As indicated by their very similar functionality both crystallographically distinct AsF6 octahedra in the structure exhibit no significant differences. Distances to non-bridging F atoms among AsF6 octahedra are well within the typical range observed in isolated AsF6 octahedra. Distances to bridging fluorines are considerably longer by about 10 pm, but with no difference for the Fe compound as compared to its Co relative. In all AsF6 moieties no angle deviates by more than 0.6° from the ideal value of 90° while slightly larger deviations are observed for the metal octahedra. Most pronounced deviations are associated with the ligands donated by the solvent. While distances of non-bridging fluorine atoms in AsF3 compare well with those in AsF6 octahedra the bridging distances are longer by about 3 pm. The lone-pairs of the As(III) ions are directed towards the rather open ring centers in neighbouring layers. Finally this arrangement results in a ABAB... type stacking sequence (Figure 2 and Figure 3).
Figure 1. The environment of   transition metal atoms in the M(AsF6)2·2AsF3 structure
with the labelling scheme. The ellipsoids are drawn at the 40% probability level.
222
Figure 2. Part of the layer in the M(AsF6)2·2AsF3 structure showing the characteristic rings.
Figure 3. Part of the  M(AsF6)2·2AsF3 type structure showing the stacking sequence of the layers and the unit cell.
223
Results and Discussion 1. Synthesis and some chemistry. Reactions between Cr, Mn, Fe, Co, Ni, Cu metals and AsF5 in AsF3 as a solvent gave solutions of solvated M2+ cations and AsF6- anions from which metal(II) hexafluoroarsenates were isolated at room temperature. In the case of Cr, Mn and Cu already known compounds CrFAsF6, Mn(AsF6)2 and CuFAsF6 were obtained [1,2,17]. In the case of Fe, Co and Ni two molecules of solvent as additional ligands were coordinated to the M(AsF6)2. These reactions proceed according to the equation:
M + nAsF5  33AsF33,2935K ®M(AsF6)2·2AsF3 + (n-3)AsF5
M = Fe, Co, Ni n > 4
The oxidation of some metals of the first row of transition elements with AsF5 in other solvents, e.g. aHF [17] and SO2 [18,19] has already been described. Lanthanoid metals can also be oxidised with AsF5 in SO2 [20], however the dissolution of some lanthanoid metals in aHF proceeds already in neat aHF leading to cations in low oxidation states (Eu2+, Yb2+, Sm2+) [17]. Preliminary experiments of oxidation of some lanthanoid metals with AsF5 in AsF3 have been carried out for comparison in the course of this work. The results show that compounds of the type Ln(AsF6)3·xAsF3 (x = 2 and/or 3) may be obtained [21]. Novel approach in our work is to use AsF3 as a solvent, a compound which is produced during these reactions. This method provides access to very clean products and can be especially useful in cases where possible higher oxidation states of the metals than (+2) have to be avoided.
The reactions of Mn, Fe, Co, Ni and Cu with AsF5 in SO2 gave in all cases compounds of the type M(AsF6)2·2SO2 (Cr was not investigated). All compounds show considerable dissociation pressure at ambient temperature and already at 323 K hexafluoroarsenates without coordinated solvent were obtained [18]. However, M(AsF6)2·2SO2 type compounds have been isolated at lower temperatures [13,19], and in the case of Co also Co(AsF6)2·4SO2 was obtained [22]. With the exception of the latter compound [23], structures of these compounds are not known. Comparison of the results with those obtained in AsF3 as a solvent shows that AsF3 is a slightly weaker ligand than SO2. In the case of Mn and Cu compounds without coordinated AsF3 were isolated at room temperature, while for Fe, Co and Ni compounds with analogous composition (M(AsF6)2·2X, X = solvent molecule) exist in both solvents. Particular compositions and related structures are clearly driven by quite subtle effects, Cr(II) and
224
Cu(II) being particularly interesting due to pronounced Jahn-Teller effect usually observed with these ions. However, only the structure of CuFAsF6 has been solved recently [24]. In the case of Fe only FeFAsF6 and Fe2F(AsF6)3 were described in the literature [2], but the structures are not known. Mn(AsF6)2 has a quite unique structure [12] which is clearly different from the yet unknown structure of the isostructural pair Co(AsF6)2 and Ni(AsF6)2 as indicated by X-ray powder diffraction patterns.
Co(AsF6)2·2AsF3 crystals were obtained by the reaction between CoF3 and supercritical AsF5 containing some aHF as catalyst. Possible route for AsF3 formation is reduction of AsF5, which permanently diffused through a Teflon liner, on the metal walls of the protective vessel. Further it is known that in the presence of AsF5 in aHF as a solvent Co(III) is reduced to Co(II) and elemental fluorine is obtained [25]. It was shown that Co(AsF6)2·2AsF3 may also be prepared by reaction of Co(AsF6)2 and AsF3. This implies that analogous reaction with isostructural Ni(AsF6)2 should also proceed. It was also found that M(AsF6)2·2AsF3 (M = Fe,Co,Ni) compounds solvolyze in aHF loosing AsF3 molecules. This is the reason why work in aHF with these systems always leads to compounds without coordinated AsF3, although it is always present as reaction product.
Further, we have been interested whether it is possible to exchange AsF3 in M(AsF6)2·2AsF3 (M = Fe, Co, Ni) by some stronger ligands, e.g. by SbF3 or SO2. The results show that it is possible to replace AsF3 in M(AsF6)2·2AsF3 compounds with SbF3 even in AsF3. So far only reactions with M = Co or Ni have been carried out and new compounds Co(AsF6)2·2SbF3 and Ni(AsF6)2·2SbF3 were synthesized using this approach. It was also shown that AsF3 is replaced by SO2 in reactions of M(AsF6)2·2AsF3 (M = Co, Ni) in liquid SO2. The analogous, already known, compounds Co(AsF6)2·2SO2 and Ni(AsF6)2·2SO2 [13,19] were obtained. This confirms previous observations that SO2 is coordinated more strongly than AsF3. Even more, in reactions where AsF3 is formed as by-product, but which were carried out in SO2 as a solvent, only adducts with SO2 are isolated [20]. The dissolution of Fe(AsF6)2·2AsF3 in liquid SO2 is quite vigorous even at temperatures below 273 K and only decomposition products without coordinating solvent have been identified.
225
140 -, 120
~   100-
c
Xi
co
80 -60 -
w      40 -o %      20 -
0
10000 -,
8000 -
C
3       6000 H
Ju
Co(AsF6)2 * 2AsF3 (cale.)
i, .,i, |jj,li,ll| frU U-l'......H'-| . ¦¦¦¦'¦ r>'' '-T
'   ¦"' ''"I
¦fi
to
c
4000
Ä       2000 -
C
M.
JUJIJL
Ni(AsF6)2 *2AsF3
i^JJ^XhJIjjA^Jj^^
|"7   i    i   "i   |    i    i    i    l    |    i    i    i    l    |   i    l   l    i   |   i   i    i   I   |   ¦    '    I    '   |    '    '    '    '    |    '    '    '    '    I    '   '    '    '    I
10            20            30            40            50            60            70            80            90           100
29 {k= 178.892 pm)
Figure 4. Powder pattern of Ni(AsF6)2·2AsF3 obtained with an image plate based Guinier camera applying Co-Kai radiation in comparison to the diagram of the Co analogue calculated from the refined single crystal structure.
2. Structure. X-ray powder diffraction diagrams of M(AsF6)2·2AsF3 for the three isolated phases indicate that they are most likely isostructural with the size of the unit cell becoming smaller going from iron to cobalt and finally to the nickel compound. While the Fe and Co complexes are clearly isostructural as proved by respective single crystal structure determinations, due to special circumstances a slight uncertainty remains for the Ni analogue. As depicted in Figure 4 the observed powder pattern of the Ni compound very closely resembles the one calculated from the refined structural model of the Co analogue. Despite the narrow line width in connection with high resolution an unambiguous indexing is prevented by accidental overlapping of prominent lines. Most likely a- and c-axis of the unit cell are almost identical for the Ni compound. In connection with the monoclinic angle rather close to 90° this makes twinning most likely for this compound.  Twinning was frequently observed even with the Fe and Co
226
compounds, thus hampering the structure solution for quite some time.
The characteristic rings within the infinite layers are well known with fluorides of octahedrally coordinated transition metals in particular with tetrafluorides. However, in the present structure all bridging fluorine ligands are replaced by AsF6- ions while the non-bridging apical positions are occupied by AsF3 molecules. The rather special situation with the transition metal as well as the centers of the AsF6- ions on centers of symmetry directly relates the distance between neighbouring layers to the b-axis of the unit cell. On the other hand this parameter resembles quite directly the bonding of the AsF3 which is clearly weaker for the Fe compound than in the Co compound. Well in accordance with this interpretation is the observation that the Fe complex looses AsF3 quite readily. However, the Fe system is more complicated as Fe(AsF6)2 is not stable in dry unsolvated form. Unsolvated Fe2+ is probably a sufficiently powerful fluoride ion acceptor to take F- from AsF6- in Fe(AsF6)2, which thus looses AsF5 and finally FeFAsF6 is formed.
The stabilities of corresponding complexes of divalent ions of the first transition series irrespective of the particular ligands involved, usually vary in the Irving-Williams [26] order which is the reverse of the order for the cation radii:
MnII<FeII<CoII<NiII<CuII>ZnII
In the case of manganese only Mn(AsF6)2 is obtained while in the case of iron, cobalt and nickel the stability of these complexes is increasing in the same direction. In the case of copper the compound Cu(AsF6)2 is not formed even in solvated form in aHF acidified by AsF5.
The structure of Co(AsF6)2·2AsF3 is closely related to the structure of H3OCo(AsF6)3 [25] in the way that the AsF3 ligands are replaced by bridging AsF6- ions. Of course, the now connected layers are stacked directly above one another and not shifted as in the M(AsF6)2·2AsF3 type compounds. On the other hand the open centers of the rings are forming channels in which the oxonium ions are located. Well in accordance with the above arguments this three-dimensional network shows typical features of an MX3 type structure as indicated by the composition.
Acknowledgement. The authors gratefully acknowledge Prof. A. Simon for providing measuring time on the diffractometer, R. Moravec for help in preparative work, B. Sedej for chemical analyses, and Slovenian-German Joint Fund and Ministry of Science and Technology of the Republic of Slovenia for financial support.
227
Supplementary material. Further details on the crystal structures may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft für wissenschaftlichtechnische Information mbH, D-76344 Eggenstein-Leopoldshafen, upon quoting the numbers CSD-410833 (Fe-compound), CSD-410834 (Co-compound), names of the authors and journal citation.
References
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Povzetek
Oksidacija prehodnih elementov prve vrste z AsF5 v AsF3 kot topilu pri sobni temperaturi: sinteza in struktura M(AsF6)2·2AsF3, M = Fe, Co, Ni
Raziskali smo novo sintezno metodo - reakcije med nekaterimi kovinami (Cr, Mn, Fe, Co, Ni in Cu) in AsF5 v AsF3 kot topilu. Ugotovili smo, da AsF5 oksidira kovine do oksidacijskega stanja (+2). V primeru Cr, Mn in Cu smo dobili enake spojine kot v superkislem sistemu MF2-AsF5-aHF (aHF = brezvodni vodikov fluorid): CrFAsF6, Mn(AsF6)2 in CuFAsF6. V primeru Fe, Co in Ni pa smo sintetizirali nov tip heksafluoroarzenatov: M(AsF6)2=2AsF3. Te spojine so izostrukturne in kristalizirajo v monoklinski prostorski skupini P21/n (št. 14) z Z=2. Dimenzije osnovne celice spojine Fe(AsF6)2=2AsF3 pri 292 K so: a = 733,8(4) pm, b = 1303,0(8) pm, c = 735,4(5) pm, 3 = 91,43(7)°, za spojino Co(AsF6)2=2AsF3 pri 288 K pa: a = 725,9(4) pm, b = 1292,9(7) pm, c = 727,6(5) pm, 3 = 91,64(5)°. Strukturo sestavljajo plasti dosežene s transkoordinacijo enot AsF6 v osnovni ravnini oktaedra okoli prehodne kovine. Oba aksialna položaja v tem oktaedru sta zasedena z molekulama AsF3. Glavna značilnost posamezne plasti so obroči, sestavljeni iz oktaedrov AsF6 in MF6, ki si delijo skupna oglišča. Molekule AsF3 sosednjih plasti so lokalizirane nad in pod središčem posameznega obroča, zato je razvrstitev plasti v ABAB… skladu.