The correct space group of NaPF₆·H₂O

The structure of sodium hexa­fluoro­phos­phate monohydrate, NaPF₆·H₂O, has been inadvertently redetermined, revealing that the previously reported space group, Imma, was assigned incorrectly, with the a and b axes interchanged. The correct space group is Pnna. The program PLATON [Spek (2003). J. Appl. Cryst. 36, 7-13] suggested both Imma and Pmma as possible space groups, but only Pnna is consistent with the systematic absences. The inter-ionic and hydrogen-bonding interactions in the lattice form a three-dimensional network.

S1. Comment

During the process of crystallization of a peptide, crystals of NaPF₆·H₂O, (I), were isolated as thin tubes. A fragment of a tube was mounted on a diffractometer for determination of the crystal structure. The unit-cell volume [505.1 (4) ų] suggested that the compound was not the expected peptide, but since the unit cell obtained was not present in the Cambridge Structural Database (CSD; Allen, 2002), a full data collection was undertaken.

The true composition of (I) was determined upon structure solution and refinement. A subsequent search of the Inorganic Crystal Structure Database (FIZ-Karlsruhe, 1999) revealed that the structure of (I) had been reported by Bode & Teufer (1956), who described an orthorhombic unit cell [a = 7.962 (5) Å, b = 10.594 (10) Å and c = 6.116 (5) Å] in space group Imma. Our orthorhombic unit cell has a different axis setting [viz. a = 10.559 (4) Å, b = 7.899 (3) Å and c = 6.057 (3) Å] and space group Pnna. It is hard to fault the original study, since those results were obtained 47 years ago and were based on 102 reflections only, while our own conclusion is based on the measurement of 4199 reflections. The program PLATON suggested the Imma space group for the unit cell, which would be inconsistent with the observed systematic absences, as the body centering is certainly absent. PLATON also suggested the Pmma space group as a possibility, but the presence of two diagonal glide planes perpendicular to the a and b axes is very clear from the data. Previously, we reported that crystallographic checking software could misinterpret systematically weak data and that caution needs to be exercised in the evaluation of the output of programs such as CHECKCIF and PLATON (Guzei et al., 2002).

In the structure of NaPF₆·H₂O (Fig. 1), the P atom occupies an inversion center while the O and Na atoms reside on twofold axes. The geometry of the PF₆⁻ anion is octahedral, with an average P—F distance of 1.597 (2) Å. In the paper by Bode & Teufer (1956), two types of P—F bonds were reported, including four equatorial bonds of 1.58 Å and two axial bonds of 1.73 Å. A CSD search for PF₆⁻ anions returned 1591 P—F bond lengths ranging between 1.303 and 1.707 Å, with the mean being 1.55 (3) Å. A density functional theory (DFT) optimization of the geometry of an O_h-symmetric PF₆⁻ anion at the B3LYP/6–311+G* level (Gaussian98, 1998) produced a P—F distance of 1.646 Å. DFT calculations tend to overestimate distances in some octahedral anions by 2–4% (Guzei, 2003), and therefore the calculated length can be adjusted as 1.60 (1) Å (0.97 x 1.646 Å). This value is in reasonable agreement with the experimental data. The P—F distance of 1.73 Å reported in the literature is much longer than the calculated value of 1.646 Å and hence is highly likely to be erroneously long.

The environment about the Na atoms is octahedral, with four shorter Na—F equatorial distances [2.293 (2) Å] and two longer axial Na—O distances [2.4012 (17) Å]. Each water molecule participates in four intermolecular interactions, two of which are symmetry independent. One is the Na···O contact just mentioned and the other is a charge-assisted interionic O—H···F hydrogen-bonding interaction, with an O···F separation of 3.064 (2) Å and an O—H···F angle spanning 168 (4)°. A CSD search for relevant hydrogen-bonding interactions returned over 50 entries; however, those with O—H···F angles below 150° were eliminated from the list. In the remaining 29 entries, the average O···F separation is 3.04 (14) Å, with the O–H···F angle averaging 160 (7)°. Thus, the interactions observed in the structure of (I) fall within the expected range.

The interionic and hydrogen-bonding interactions in the lattice of (I) form a three-dimensional network, as shown in Fig. 2.

S2. Refinement

The O—H distance was constrained to 0.880 (1) Å, while the positional and thermal parameters of the H atom were allowed to refine.

Computing details

Data collection: SMART (Bruker, 2000–2003); cell refinement: SMART; data reduction: SAINT (Bruker, 2000–2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000–2003); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A partial view of the NaPF6·H2O structure. The H···F3 hydrogen bonds are shown as dashed lines.
	Fig. 2. The crystal lattice of NaPF6·H2O, viewed along the c axis. Hydrogen bonds have been omitted for clarity.

sodium hexafluorophospate monohydrate

Crystal data

NaPF6·H2O	F(000) = 360
Mr = 185.98	Dx = 2.446 Mg m^−3
Orthorhombic, Pnna	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2a 2bc	Cell parameters from 741 reflections
a = 10.559 (4) Å	θ = 2–25°
b = 7.898 (3) Å	µ = 0.69 mm^−1
c = 6.057 (3) Å	T = 100 K
V = 505.1 (4) Å^3	Needle, colorless
Z = 4	0.43 × 0.32 × 0.26 mm

Data collection

Bruker CCD 1000 area-detector diffractometer	520 independent reflections
Radiation source: fine-focus sealed tube	449 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.046
ω scan	θmax = 26.4°, θmin = 4.6°
Absorption correction: multi-scan (SADABS; Bruker, 2000–2003)	h = −13→13
Tmin = 0.756, Tmax = 0.841	k = −9→9
3685 measured reflections	l = −7→7

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)] = 0.045	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.098	H atoms treated by a mixture of independent and constrained refinement
S = 1.18	w = 1/[σ^2(Fo^2) + (0.0347P)^2 + 0.8605P] where P = (Fo^2 + 2Fc^2)/3
520 reflections	(Δ/σ)max < 0.001
48 parameters	Δρmax = 0.38 e Å^−3
1 restraint	Δρmin = −0.52 e Å^−3

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
P	0.5000	1.0000	0.0000	0.0106 (3)
F1	0.60126 (17)	0.8518 (2)	0.0330 (3)	0.0246 (5)
F2	0.38721 (16)	0.8660 (2)	0.0199 (3)	0.0263 (5)
F3	0.49622 (15)	1.0293 (2)	0.2604 (3)	0.0224 (5)
O	0.7500	0.5000	0.0253 (4)	0.0145 (7)
H	0.686 (2)	0.503 (4)	−0.068 (5)	0.038 (11)*
Na	0.76208 (13)	0.7500	0.2500	0.0144 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P	0.0083 (5)	0.0126 (5)	0.0109 (5)	−0.0005 (4)	−0.0021 (4)	−0.0010 (4)
F1	0.0236 (10)	0.0227 (10)	0.0274 (10)	0.0128 (8)	−0.0103 (8)	−0.0053 (8)
F2	0.0214 (10)	0.0278 (11)	0.0298 (11)	−0.0132 (8)	−0.0054 (8)	0.0054 (8)
F3	0.0216 (9)	0.0341 (10)	0.0116 (8)	0.0051 (7)	−0.0019 (8)	−0.0054 (7)
O	0.0110 (14)	0.0186 (15)	0.0138 (14)	0.0011 (12)	0.000	0.000
Na	0.0153 (8)	0.0136 (8)	0.0143 (8)	0.000	0.000	0.0021 (6)

Geometric parameters (Å, º)

P—F3^i	1.5948 (19)	O—Na^iii	2.4012 (17)
P—F3	1.5948 (19)	O—H	0.88 (3)
P—F1	1.5977 (17)	Na—F2^iv	2.293 (2)
P—F1^i	1.5977 (17)	Na—F2^v	2.293 (2)
P—F2^i	1.5980 (17)	Na—F1^vi	2.293 (2)
P—F2	1.5980 (17)	Na—O^vii	2.4012 (17)
F1—Na	2.293 (2)	Na—Na^iii	3.9570 (17)
F2—Na^ii	2.2929 (19)	Na—Na^viii	3.9570 (16)
O—Na	2.4012 (17)
H—O—H^iii	100 (5)	F2^v—Na—F1^vi	167.30 (8)
F3^i—P—F3	180.0	F1—Na—F1^vi	84.43 (11)
F3^i—P—F1	90.04 (9)	F2^iv—Na—O	96.10 (7)
F3—P—F1	89.96 (9)	F2^v—Na—O	87.42 (7)
F3^i—P—F1^i	89.96 (9)	F1—Na—O	85.66 (6)
F3—P—F1^i	90.04 (9)	F1^vi—Na—O	89.83 (7)
F1—P—F1^i	180.0	F2^iv—Na—O^vii	87.42 (7)
F3^i—P—F2^i	90.16 (9)	F2^v—Na—O^vii	96.10 (7)
F3—P—F2^i	89.84 (9)	F1—Na—O^vii	89.83 (7)
F1—P—F2^i	89.76 (11)	F1^vi—Na—O^vii	85.66 (6)
F1^i—P—F2^i	90.24 (11)	O—Na—O^vii	173.91 (7)
F3^i—P—F2	89.84 (9)	F2^iv—Na—Na^iii	68.81 (5)
F3—P—F2	90.16 (9)	F2^v—Na—Na^iii	115.83 (6)
F1—P—F2	90.24 (11)	F1—Na—Na^iii	107.59 (6)
F1^i—P—F2	89.76 (11)	F1^vi—Na—Na^iii	66.56 (5)
F2^i—P—F2	180.0	O—Na—Na^iii	34.52 (6)
P—F1—Na	145.52 (10)	O^vii—Na—Na^iii	144.80 (6)
P—F2—Na^ii	129.81 (10)	F2^iv—Na—Na^viii	115.83 (6)
Na—O—Na^iii	110.96 (11)	F2^v—Na—Na^viii	68.81 (5)
Na—O—H	113 (2)	F1—Na—Na^viii	66.56 (5)
Na^iii—O—H	110 (2)	F1^vi—Na—Na^viii	107.59 (6)
F2^iv—Na—F2^v	109.63 (11)	O—Na—Na^viii	144.80 (6)
F2^iv—Na—F1	167.30 (8)	O^vii—Na—Na^viii	34.52 (6)
F2^v—Na—F1	82.99 (8)	Na^iii—Na—Na^viii	172.61 (8)
F2^iv—Na—F1^vi	82.99 (8)
F3^i—P—F1—Na	151.3 (2)	P—F1—Na—F1^vi	81.1 (2)
F3—P—F1—Na	−28.7 (2)	P—F1—Na—O	171.4 (2)
F2^i—P—F1—Na	61.2 (2)	P—F1—Na—O^vii	−4.5 (2)
F2—P—F1—Na	−118.8 (2)	P—F1—Na—Na^iii	144.36 (18)
F3^i—P—F2—Na^ii	−13.49 (14)	P—F1—Na—Na^viii	−30.80 (19)
F3—P—F2—Na^ii	166.51 (14)	Na^iii—O—Na—F2^iv	37.17 (5)
F1—P—F2—Na^ii	−103.53 (14)	Na^iii—O—Na—F2^v	146.64 (7)
F1^i—P—F2—Na^ii	76.47 (14)	Na^iii—O—Na—F1	−130.19 (7)
P—F1—Na—F2^iv	72.9 (4)	Na^iii—O—Na—F1^vi	−45.76 (5)
P—F1—Na—F2^v	−100.7 (2)	Na^iii—O—Na—Na^viii	−167.10 (14)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O—H···F3^ix	0.88 (3)	2.20 (1)	3.064 (2)	168 (4)

Symmetry code: (ix) −x+1, y−1/2, z−1/2.