Redetermination of Na₃TaF₈

The crystal structure of tris­odium octa­fluoridotantalate, Na₃TaF₈, has been redetermined using diffractometer data collected at 153 K, resulting in more accurate bond distances and angles than obtained from a previous structure determination based on film data. The structure is built from layers running along [101], which are formed by distorted [TaF₈] anti­prisms and [NaF₆] recta­ngular bipyramids sharing edges and corners. The individual layers are separated by eight-coordinated Na ions. Two atoms in the asymmetric unit are in special positions: the Ta atom is on a twofold axis in Wyckoff position 4e and one of the Na ions lies on an inversion centre in Wyckoff site 4d.

S1. Comment

As part of our structure investigations of compounds containing Ta–F bonds (Langer et al., 2006; Smrčok et al., 2008; Smrčok et al., 2010), we present a new structure determination providing well determined atomic coordinates essential for future spectral and quantum chemistry studies. The structure of the title compound was reported to be solved in a face-centered pseudo-orthorhombic cell (though the correct monoclinic space group C2/c was recognized) using Patterson and Fourier techniques from the H0L and HK0 projections obtained from the film data, but with no subsequent refinement carried out (Hoard et al., 1954). The coordinates were given for both pseudo-orthorhombic and monoclinic cells, but unfortunately those in the true monoclinic space group are incorrect, as authors did not realise that there are different origins of coordinate systems. Monoclinic coordinates (Hoard et al., 1954) transform approximately to equivalent solution as presented here by transformation of the unit cell -a, -b, a+c with a shift of origin by 1/4, 3/4, 1/2.

The structure of Na₃TaF₈ can be regarded as consisting of layers built by sharing cornes and/or edges of the [TaF₈] antiprisms (Ta1 is at twofold axis in position 4e) and the deformed [NaF₆] rectangular bipyramids (Na1 is at 1 in position 4d) (Fig. 1). The Na2 atoms, which like all the F atoms appear in the general positions of the C2/c space group, are sandwiched between the layers and their formal coordination number is eight. The individual Ta—F bond distances in the antiprism (Table 1 and Fig. 2) are longer than those in the monocapped [TaF₇] trigonal prisms in K₂TaF₇ [1.919 (3)–1.976 (2) Å; Torardi & Brixner, 1987], but comparable to the Ta—F bond distances in the [TaF₇] prism in K₃TaF₈ [density functional theory (DFT) data: 1.964–1.995 Å; Smrčok et al., 2010]. The longest Ta—F bond distances involve F1 and F4 shared by the shorter basal edge of the Na1 polyhedron (Fig. 3). Somewhat shorter Ta—F bonds involve F3 atoms, which are shared by the corners of Na1–F bipyramids and [TaF₇] antiprism, while the shortest Ta—F bonds involve F2 atoms engaged only in a bond to the interlayer Na2 atom. This variation in the Ta—F bond lentghs is of the same order as in the antiprisms of IF₈^-, WF₈^2- and ReF₈^2-, but is remarkably smaller than that in XeF₈^2- (Adam et al., 1996). The contact F—F distances within the bases of the antiprism (Fig. 2) are slightly shorter than those separating the atoms of the respective bases, i.e. 2.469 (5) Å [F3—F4 and F3ⁱ—F4ⁱ; symmetry code: (i) 1-x, y, 3/2-z] or 2.484 (4) Å (F1—F2 and F1ⁱ—F2ⁱ).

The values of Na—F distances appear on the both sides of the reference value of 2.317 Å reported for NaF (Deshpande, 1961), but the majority of them are longer. Because all of the distances are shorter than sum of the van der Waals radii for Na and F (3.74 Å), they can be considered to be the bond distances. The coordination polyhedron around the hexacoordinate Na1 atom is the distorted rectangular bipyramid with the two sets of Na—F bond distances (Table 1). The polyhedron around the Na2 atom is a highly distorted cube with the span of the Na—F distances larger (0.350 Å) than in the polyhedron around the Na1 atom (0.208 Å).

Related literature

For related literature, see: Adam et al. (1996); Deshpande (1961); Hoard et al. (1954); Langer et al. (2006); Smrčok et al. (2008, 2010); Torardi & Brixner (1987).

S2. Experimental

A solution of HF [weight fraction (w) = 5.4 × 10^-4, 327.5 ml] was prepared by dilution of concentrated HF (w = 0.40, 0.5 ml) into distilled water. This solution (30 ml) was used as solvent for NaF (0.152 g, 3.62 mmol) and TaF₅ (0.25 g, 0.91 mmol). The latter compound was weighted in a dry-box into a hermetically sealed transport ampoule before dissolving it in HF solution. This solution was let to evaporate spontaneously at room temperature for 5 d. When the solid phase had formed in a sufficient amount, it was filtered on a paper filter and washed with acetone and dried under vacuum at 333 K for 30 min. All experiments were carried out using platinum or plastic dishes.

S3. Refinement

All atoms were refined anisotropically without any restraints.

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003) and SADABS (Sheldrick, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: PLATON (Spek, 2009).

Figures

	Fig. 1. Polyhedral representation of the title structure, viewed along the b axis. The [TaF8] polyhedra are dark (dark green in the electronic version of the paper), the [NaF6] rectangular bipyramids around Na1 atoms are light grey (yellow) and the Na2 atoms are depicted as large spheres.
	Fig. 2. The numbering scheme for the title compound, with atomic displacement ellipsoids drawn at the 50% probability level. Slightly deformed F2/F3/F2^i/F3^i and F1/F4/F1^i/F4^i squares form the upper and lower bases of the distorted [TaF8] antiprism. The F···F seprations in the lower and upper base are 2.354 (5)/2.415 (4) and 2.365 (6)/2.406 (3) Å, respectively. [Symmetry code: (i) 1-x, y, 3/2-z.]
	Fig. 3. The layer consisting of [TaF8] antiprisms joined to [NaF6] polyhedra.

trisodium octafluoridotantalate

Crystal data

Na3TaF8	F(000) = 712
Mr = 401.92	Dx = 4.531 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2547 reflections
a = 11.5203 (14) Å	θ = 4.1–32.3°
b = 5.3453 (6) Å	µ = 18.98 mm^−1
c = 11.1540 (13) Å	T = 153 K
β = 120.919 (2)°	Prism, colourless
V = 589.25 (12) Å^3	0.24 × 0.18 × 0.07 mm
Z = 4

Data collection

Siemens CCD area-detector diffractometer	1030 independent reflections
Radiation source: fine-focus sealed tube	961 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.045
ω scans	θmax = 32.9°, θmin = 4.1°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	h = −16→16
Tmin = 0.092, Tmax = 0.350	k = −8→7
4688 measured reflections	l = −16→16

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	w = 1/[σ^2(Fo^2) + (0.0288P)^2 + 0.1143P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.051	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 1.93 e Å^−3
1030 reflections	Δρmin = −3.00 e Å^−3
58 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0094 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ta1	0.5000	0.68650 (4)	0.7500	0.00503 (10)
Na1	0.7500	0.2500	1.0000	0.0149 (6)
Na2	0.6346 (2)	0.7569 (4)	1.1027 (2)	0.0110 (4)
F1	0.5578 (3)	0.4657 (5)	0.9162 (3)	0.0095 (5)
F2	0.4382 (3)	0.8740 (6)	0.8603 (3)	0.0140 (6)
F3	0.6553 (3)	0.8921 (6)	0.8892 (3)	0.0145 (6)
F4	0.6666 (3)	0.4915 (5)	0.7855 (3)	0.0091 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ta1	0.00306 (13)	0.00520 (14)	0.00600 (13)	0.000	0.00174 (9)	0.000
Na1	0.0082 (13)	0.0089 (11)	0.0173 (13)	0.0029 (10)	−0.0008 (11)	−0.0021 (10)
Na2	0.0091 (9)	0.0111 (8)	0.0128 (8)	−0.0008 (7)	0.0057 (8)	−0.0010 (7)
F1	0.0070 (12)	0.0109 (12)	0.0101 (11)	0.0018 (10)	0.0040 (10)	0.0036 (10)
F2	0.0125 (14)	0.0157 (13)	0.0161 (12)	0.0037 (11)	0.0091 (11)	−0.0040 (11)
F3	0.0086 (13)	0.0148 (13)	0.0151 (12)	−0.0023 (11)	0.0025 (11)	−0.0044 (11)
F4	0.0043 (11)	0.0129 (13)	0.0119 (11)	0.0020 (10)	0.0053 (10)	−0.0016 (10)

Geometric parameters (Å, º)

Ta1—F1	2.002 (3)	Na1—Na2	3.462 (2)
Ta1—F1^i	2.002 (3)	Na2—F1	2.375 (3)
Ta1—F2^i	1.981 (3)	Na2—F1^v	2.428 (3)
Ta1—F2	1.981 (3)	Na2—F2	2.557 (4)
Ta1—F3	1.991 (3)	Na2—F2^vi	2.264 (4)
Ta1—F3^i	1.991 (3)	Na2—F3	2.614 (4)
Ta1—F4	2.036 (3)	Na2—F3^iii	2.506 (4)
Ta1—F4^i	2.036 (3)	Na2—F4^vii	2.299 (3)
Na1—F1	2.231 (3)	Na2—F4^iii	2.382 (3)
Na1—F1^ii	2.231 (3)	Na2—Na1^viii	3.405 (2)
Na1—F3^iii	2.234 (3)	F1—Na2^v	2.428 (3)
Na1—F3^iv	2.234 (3)	F2—Na2^vi	2.263 (4)
Na1—F4	2.439 (3)	F3—Na1^viii	2.234 (3)
Na1—F4^ii	2.439 (3)	F3—Na2^iii	2.506 (4)
Na1—Na2^iii	3.405 (2)	F4—Na2^ix	2.299 (3)
Na1—Na2^iv	3.405 (2)	F4—Na2^iii	2.382 (3)
Na1—Na2^ii	3.462 (2)
F2^i—Ta1—F2	119.21 (18)	Na2^ii—Na1—Na2	180.00 (8)
F2^i—Ta1—F3	74.89 (13)	F1^ii—Na1—Ta1^ii	28.85 (7)
F2—Ta1—F3	72.66 (13)	F1—Na1—Ta1^ii	151.15 (7)
F2^i—Ta1—F3^i	72.66 (13)	F3^iii—Na1—Ta1^ii	98.95 (8)
F2—Ta1—F3^i	74.90 (12)	F3^iv—Na1—Ta1^ii	81.05 (8)
F3—Ta1—F3^i	112.98 (18)	F4—Na1—Ta1^ii	147.99 (6)
F2^i—Ta1—F1	144.98 (12)	F4^ii—Na1—Ta1^ii	32.01 (6)
F2—Ta1—F1	77.16 (12)	Na2^iii—Na1—Ta1^ii	117.50 (3)
F3—Ta1—F1	82.25 (12)	Na2^iv—Na1—Ta1^ii	62.50 (3)
F3^i—Ta1—F1	141.81 (12)	Na2^ii—Na1—Ta1^ii	57.65 (3)
F2^i—Ta1—F1^i	77.16 (12)	Na2—Na1—Ta1^ii	122.35 (3)
F2—Ta1—F1^i	144.98 (12)	F1^ii—Na1—Ta1	151.15 (7)
F3—Ta1—F1^i	141.81 (12)	F1—Na1—Ta1	28.85 (7)
F3^i—Ta1—F1^i	82.25 (12)	F3^iii—Na1—Ta1	81.05 (8)
F1—Ta1—F1^i	107.72 (16)	F3^iv—Na1—Ta1	98.95 (8)
F2^i—Ta1—F4	77.07 (12)	F4—Na1—Ta1	32.01 (6)
F2—Ta1—F4	137.87 (11)	F4^ii—Na1—Ta1	147.99 (6)
F3—Ta1—F4	75.61 (12)	Na2^iii—Na1—Ta1	62.50 (3)
F3^i—Ta1—F4	144.46 (12)	Na2^iv—Na1—Ta1	117.50 (3)
F1—Ta1—F4	71.71 (11)	Na2^ii—Na1—Ta1	122.35 (3)
F1^i—Ta1—F4	73.15 (10)	Na2—Na1—Ta1	57.65 (3)
F2^i—Ta1—F4^i	137.87 (11)	Ta1^ii—Na1—Ta1	179.999 (5)
F2—Ta1—F4^i	77.07 (12)	F2^vi—Na2—F4^vii	104.32 (13)
F3—Ta1—F4^i	144.46 (12)	F2^vi—Na2—F1	136.45 (13)
F3^i—Ta1—F4^i	75.61 (12)	F4^vii—Na2—F1	102.47 (12)
F1—Ta1—F4^i	73.15 (10)	F2^vi—Na2—F4^iii	76.07 (12)
F1^i—Ta1—F4^i	71.71 (11)	F4^vii—Na2—F4^iii	100.90 (9)
F4—Ta1—F4^i	118.43 (16)	F1—Na2—F4^iii	130.82 (13)
F2^i—Ta1—Na2^i	47.69 (9)	F2^vi—Na2—F1^v	92.04 (13)
F2—Ta1—Na2^i	124.20 (9)	F4^vii—Na2—F1^v	61.14 (10)
F3—Ta1—Na2^i	121.98 (10)	F1—Na2—F1^v	71.94 (12)
F3^i—Ta1—Na2^i	49.40 (9)	F4^iii—Na2—F1^v	155.75 (13)
F1—Ta1—Na2^i	150.09 (9)	F2^vi—Na2—F3^iii	135.43 (14)
F1^i—Ta1—Na2^i	42.49 (9)	F4^vii—Na2—F3^iii	94.17 (12)
F4—Ta1—Na2^i	96.02 (8)	F1—Na2—F3^iii	75.01 (11)
F4^i—Ta1—Na2^i	90.43 (8)	F4^iii—Na2—F3^iii	60.62 (11)
F2^i—Ta1—Na2	124.20 (9)	F1^v—Na2—F3^iii	132.03 (13)
F2—Ta1—Na2	47.69 (9)	F2^vi—Na2—F2	76.70 (12)
F3—Ta1—Na2	49.40 (10)	F4^vii—Na2—F2	136.98 (13)
F3^i—Ta1—Na2	121.98 (10)	F1—Na2—F2	60.34 (10)
F1—Ta1—Na2	42.49 (9)	F4^iii—Na2—F2	120.37 (13)
F1^i—Ta1—Na2	150.09 (9)	F1^v—Na2—F2	75.85 (11)
F4—Ta1—Na2	90.43 (8)	F3^iii—Na2—F2	115.43 (12)
F4^i—Ta1—Na2	96.02 (8)	F2^vi—Na2—F3	98.10 (13)
Na2^i—Ta1—Na2	167.40 (7)	F4^vii—Na2—F3	156.87 (14)
F2^i—Ta1—Na1	113.86 (9)	F1—Na2—F3	63.30 (10)
F2—Ta1—Na1	104.11 (9)	F4^iii—Na2—F3	78.91 (11)
F3—Ta1—Na1	73.53 (9)	F1^v—Na2—F3	124.24 (11)
F3^i—Ta1—Na1	172.29 (9)	F3^iii—Na2—F3	65.30 (12)
F1—Ta1—Na1	32.54 (8)	F2—Na2—F3	54.12 (10)
F1^i—Ta1—Na1	95.01 (8)	F2^vi—Na2—Na1^viii	68.41 (10)
F4—Ta1—Na1	39.42 (7)	F4^vii—Na2—Na1^viii	146.44 (10)
F4^i—Ta1—Na1	96.70 (7)	F1—Na2—Na1^viii	104.29 (9)
Na2^i—Ta1—Na1	131.49 (3)	F4^iii—Na2—Na1^viii	45.76 (7)
Na2—Ta1—Na1	58.55 (3)	F1^v—Na2—Na1^viii	147.89 (10)
F2^i—Ta1—Na1^i	104.11 (9)	F3^iii—Na2—Na1^viii	73.81 (9)
F2—Ta1—Na1^i	113.86 (9)	F2—Na2—Na1^viii	74.98 (9)
F3—Ta1—Na1^i	172.29 (9)	F3—Na2—Na1^viii	41.00 (7)
F3^i—Ta1—Na1^i	73.53 (9)	F2^vi—Na2—Ta1	108.26 (10)
F1—Ta1—Na1^i	95.01 (8)	F4^vii—Na2—Ta1	136.57 (10)
F1^i—Ta1—Na1^i	32.54 (8)	F1—Na2—Ta1	34.70 (7)
F4—Ta1—Na1^i	96.70 (7)	F4^iii—Na2—Ta1	114.13 (9)
F4^i—Ta1—Na1^i	39.42 (7)	F1^v—Na2—Ta1	89.53 (8)
Na2^i—Ta1—Na1^i	58.55 (4)	F3^iii—Na2—Ta1	82.26 (8)
Na2—Ta1—Na1^i	131.49 (4)	F2—Na2—Ta1	34.96 (7)
Na1—Ta1—Na1^i	100.311 (10)	F3—Na2—Ta1	35.33 (7)
F1^ii—Na1—F1	180.000 (1)	Na1^viii—Na2—Ta1	73.94 (4)
F1^ii—Na1—F3^iii	96.50 (11)	F2^vi—Na2—Na1	169.70 (11)
F1—Na1—F3^iii	83.50 (11)	F4^vii—Na2—Na1	85.85 (9)
F1^ii—Na1—F3^iv	83.50 (10)	F1—Na2—Na1	39.72 (7)
F1—Na1—F3^iv	96.50 (11)	F4^iii—Na2—Na1	100.70 (9)
F3^iii—Na1—F3^iv	180.0	F1^v—Na2—Na1	94.38 (9)
F1^ii—Na1—F4	119.34 (9)	F3^iii—Na2—Na1	40.09 (7)
F1—Na1—F4	60.66 (9)	F2—Na2—Na1	97.07 (9)
F3^iii—Na1—F4	85.62 (10)	F3—Na2—Na1	71.62 (8)
F3^iv—Na1—F4	94.38 (10)	Na1^viii—Na2—Na1	102.23 (5)
F1^ii—Na1—F4^ii	60.66 (9)	Ta1—Na2—Na1	63.80 (4)
F1—Na1—F4^ii	119.34 (9)	F2^vi—Na2—Ta1^v	101.58 (10)
F3^iii—Na1—F4^ii	94.38 (10)	F4^vii—Na2—Ta1^v	30.44 (7)
F3^iv—Na1—F4^ii	85.62 (10)	F1—Na2—Ta1^v	85.15 (9)
F4—Na1—F4^ii	180.000 (1)	F4^iii—Na2—Ta1^v	130.34 (10)
F1^ii—Na1—Na2^iii	93.78 (8)	F1^v—Na2—Ta1^v	30.84 (6)
F1—Na1—Na2^iii	86.22 (8)	F3^iii—Na2—Ta1^v	113.78 (10)
F3^iii—Na1—Na2^iii	50.15 (9)	F2—Na2—Ta1^v	106.57 (10)
F3^iv—Na1—Na2^iii	129.85 (9)	F3—Na2—Ta1^v	147.96 (10)
F4—Na1—Na2^iii	44.38 (7)	Na1^viii—Na2—Ta1^v	169.48 (7)
F4^ii—Na1—Na2^iii	135.62 (7)	Ta1—Na2—Ta1^v	113.49 (5)
F1^ii—Na1—Na2^iv	86.22 (8)	Na1—Na2—Ta1^v	87.99 (5)
F1—Na1—Na2^iv	93.78 (8)	Ta1—F1—Na1	118.61 (12)
F3^iii—Na1—Na2^iv	129.85 (9)	Ta1—F1—Na2	102.81 (13)
F3^iv—Na1—Na2^iv	50.15 (9)	Na1—F1—Na2	97.41 (10)
F4—Na1—Na2^iv	135.62 (7)	Ta1—F1—Na2^v	110.73 (12)
F4^ii—Na1—Na2^iv	44.38 (7)	Na1—F1—Na2^v	116.65 (12)
Na2^iii—Na1—Na2^iv	180.0	Na2—F1—Na2^v	108.06 (12)
F1^ii—Na1—Na2^ii	42.87 (8)	Ta1—F2—Na2^vi	147.08 (17)
F1—Na1—Na2^ii	137.13 (8)	Ta1—F2—Na2	97.35 (12)
F3^iii—Na1—Na2^ii	133.76 (9)	Na2^vi—F2—Na2	103.30 (12)
F3^iv—Na1—Na2^ii	46.24 (9)	Ta1—F3—Na1^viii	153.10 (16)
F4—Na1—Na2^ii	96.56 (7)	Ta1—F3—Na2^iii	108.47 (13)
F4^ii—Na1—Na2^ii	83.44 (7)	Na1^viii—F3—Na2^iii	93.67 (12)
Na2^iii—Na1—Na2^ii	102.23 (5)	Ta1—F3—Na2	95.27 (13)
Na2^iv—Na1—Na2^ii	77.77 (5)	Na1^viii—F3—Na2	88.85 (11)
F1^ii—Na1—Na2	137.13 (8)	Na2^iii—F3—Na2	114.70 (12)
F1—Na1—Na2	42.87 (8)	Ta1—F4—Na2^ix	114.67 (12)
F3^iii—Na1—Na2	46.24 (9)	Ta1—F4—Na2^iii	111.69 (13)
F3^iv—Na1—Na2	133.76 (9)	Na2^ix—F4—Na2^iii	117.61 (12)
F4—Na1—Na2	83.44 (7)	Ta1—F4—Na1	108.57 (11)
F4^ii—Na1—Na2	96.56 (7)	Na2^ix—F4—Na1	111.53 (12)
Na2^iii—Na1—Na2	77.77 (5)	Na2^iii—F4—Na1	89.85 (10)
Na2^iv—Na1—Na2	102.23 (5)

Symmetry codes: (i) −x+1, y, −z+3/2; (ii) −x+3/2, −y+1/2, −z+2; (iii) −x+3/2, −y+3/2, −z+2; (iv) x, y−1, z; (v) −x+1, −y+1, −z+2; (vi) −x+1, −y+2, −z+2; (vii) x, −y+1, z+1/2; (viii) x, y+1, z; (ix) x, −y+1, z−1/2.