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As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, the previously unknown structure of NaIO_{2}F_{2} corresponds to a new structure type based on isolated IO_{2}F_{2}^{} polyhedra forming sheets separated by Na^{+} layers. The sodium ion is octahedrally coordinated with 2/m site symmetry, while the I^{V} atom has m2m symmetry with a stereochemically active lone electron pair. The O and F atoms (both with m symmetry) are bonded to the I^{V} atoms in a fully ordered manner. A comparison with the structure of ferroelastic KIO_{2}F_{2} and with structures based on hexagonal close packing of anions, mainly rutiletype and FeTeO_{3}Ftype, reveals differences that are attributed to the smaller ionic radius of Na^{+} and the ordering of the Na and I cations.
Supporting information
NaIO_{2}F_{2} was prepared by progressive evaporation at 373 K of a 2:1 molar mixture of NaF and I_{2}O_{5} dissolved in hydrofluoric acid (40%) in a Teflon beaker. After full evaporation, transparent airstable single crystals suitable for Xray studies were obtained, growing on the surface of a second pinkcoloured amorphous phase.
Data collection: COLLECT (Nonius, 1998); cell refinement: DIRAX/LSQ (Duisenberg, 1992); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
Sodium iodine(V) oxyfluoride
top
Crystal data top
NaIO_{2}F_{2}  F(000) = 392 
M_{r} = 219.89  D_{x} = 3.943 Mg m^{−}^{3} 
Orthorhombic, Cmcm  Mo Kα radiation, λ = 0.71073 Å 
Hall symbol: C 2c 2  Cell parameters from 3200 reflections 
a = 6.9287 (10) Å  θ = 4.9–30.0° 
b = 7.2735 (13) Å  µ = 8.65 mm^{−}^{1} 
c = 7.3503 (13) Å  T = 293 K 
V = 370.42 (11) Å^{3}  Irregular tablet, colourless 
Z = 4  0.10 × 0.10 × 0.02 mm 
Data collection top
Nonius KappaCCD diffractometer  312 independent reflections 
Radiation source: finefocus sealed tube  303 reflections with I > 2σ(I) 
Graphite monochromator  R_{int} = 0.021 
Detector resolution: 9 pixels mm^{1}  θ_{max} = 30.0°, θ_{min} = 4.9° 
CCD scans  h = −9→9 
Absorption correction: multiscan (SADABS; Bruker 2001)  k = −10→10 
T_{min} = 0.478, T_{max} = 0.846  l = −10→10 
3335 measured reflections  
Refinement top
Refinement on F^{2}  Primary atom site location: structureinvariant direct methods 
Leastsquares matrix: full  Secondary atom site location: difference Fourier map 
R[F^{2} > 2σ(F^{2})] = 0.010  w = 1/[σ^{2}(F_{o}^{2}) + (0.0157P)^{2} + 2.0799P] where P = (F_{o}^{2} + 2F_{c}^{2})/3 
wR(F^{2}) = 0.023  (Δ/σ)_{max} < 0.001 
S = 0.75  Δρ_{max} = 0.40 e Å^{−}^{3} 
312 reflections  Δρ_{min} = −0.57 e Å^{−}^{3} 
22 parameters  Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^{*}=kFc[1+0.001xFc^{2}λ^{3}/sin(2θ)]^{1/4} 
0 restraints  Extinction coefficient: 0.0188 (6) 
Crystal data top
NaIO_{2}F_{2}  V = 370.42 (11) Å^{3} 
M_{r} = 219.89  Z = 4 
Orthorhombic, Cmcm  Mo Kα radiation 
a = 6.9287 (10) Å  µ = 8.65 mm^{−}^{1} 
b = 7.2735 (13) Å  T = 293 K 
c = 7.3503 (13) Å  0.10 × 0.10 × 0.02 mm 
Data collection top
Nonius KappaCCD diffractometer  312 independent reflections 
Absorption correction: multiscan (SADABS; Bruker 2001)  303 reflections with I > 2σ(I) 
T_{min} = 0.478, T_{max} = 0.846  R_{int} = 0.021 
3335 measured reflections  
Refinement top
R[F^{2} > 2σ(F^{2})] = 0.010  22 parameters 
wR(F^{2}) = 0.023  0 restraints 
S = 0.75  Δρ_{max} = 0.40 e Å^{−}^{3} 
312 reflections  Δρ_{min} = −0.57 e Å^{−}^{3} 
Special details top
Experimental. Intensity data were collected with a Nonius KappaCCD diffractometer using a monochromated Mo—Kα radiation. These intensities were corrected for absorption effects by using a multiscan method (SADABS, Bruker 2001). Structure solution by direct methods in the Cmcm space group, followed by refinement of atomic coordinates and anisotropic thermal parameters, were performed using respectively the SHELXS97 and SHELXL97 programs (Sheldrick, 2008). 
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 F^{2} against ALL reflections. The weighted Rfactor wR and goodness of fit S are based on F^{2}, conventional Rfactors R are based on F, with F set to zero for negative F^{2}. The threshold expression of F^{2} > σ(F^{2}) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. Rfactors based on F^{2} 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 (Å^{2}) top  x  y  z  U_{iso}*/U_{eq}  
I1  0.5000  0.03831 (3)  0.2500  0.01246 (9)  
Na1  0.0000  0.0000  0.0000  0.0199 (3)  
O1  0.5000  0.1881 (2)  0.0612 (2)  0.0216 (4)  
F1  0.2108 (2)  0.0372 (2)  0.2500  0.0296 (4)  
Atomic displacement parameters (Å^{2}) top  U^{11}  U^{22}  U^{33}  U^{12}  U^{13}  U^{23} 
I1  0.01676 (12)  0.01230 (13)  0.00831 (11)  0.000  0.000  0.000 
Na1  0.0251 (7)  0.0171 (6)  0.0173 (7)  0.000  0.000  0.0018 (5) 
O1  0.0361 (10)  0.0156 (7)  0.0130 (8)  0.000  0.000  0.0040 (7) 
F1  0.0184 (8)  0.0444 (11)  0.0261 (8)  0.0052 (7)  0.000  0.000 
Geometric parameters (Å, º) top
I1—O1^{i}  1.7647 (17)  Na1—F1^{vii}  2.3627 (11) 
I1—O1  1.7647 (17)  Na1—F1  2.3627 (11) 
I1—F1  2.0040 (17)  Na1—F1^{viii}  2.3627 (11) 
I1—F1^{ii}  2.0040 (17)  Na1—F1^{iii}  2.3627 (11) 
I1—O1^{iii}  2.8185 (19)  Na1—Na1^{ix}  3.6752 (6) 
I1—O1^{iv}  2.8185 (19)  Na1—Na1^{x}  3.6752 (6) 
Na1—O1^{v}  2.3124 (18)  O1—Na1^{xi}  2.3124 (18) 
Na1—O1^{vi}  2.3124 (18)  F1—Na1^{x}  2.3627 (11) 
   
O1^{i}—I1—O1  103.72 (12)  F1—Na1—F1^{viii}  76.35 (7) 
O1^{i}—I1—F1  90.14 (3)  O1^{v}—Na1—F1^{iii}  92.23 (6) 
O1—I1—F1  90.14 (3)  O1^{vi}—Na1—F1^{iii}  87.77 (6) 
O1^{i}—I1—F1^{ii}  90.14 (3)  F1^{vii}—Na1—F1^{iii}  76.35 (7) 
O1—I1—F1^{ii}  90.14 (3)  F1—Na1—F1^{iii}  103.65 (7) 
F1—I1—F1^{ii}  179.55 (10)  F1^{viii}—Na1—F1^{iii}  180.0 
O1^{i}—I1—O1^{iii}  177.62 (6)  O1^{v}—Na1—Na1^{ix}  101.21 (4) 
O1—I1—O1^{iii}  73.90 (8)  O1^{vi}—Na1—Na1^{ix}  78.79 (4) 
F1—I1—O1^{iii}  89.87 (3)  F1^{vii}—Na1—Na1^{ix}  38.95 (3) 
F1^{ii}—I1—O1^{iii}  89.87 (3)  F1—Na1—Na1^{ix}  141.05 (3) 
O1^{i}—I1—O1^{iv}  73.90 (8)  F1^{viii}—Na1—Na1^{ix}  141.05 (3) 
O1—I1—O1^{iv}  177.62 (6)  F1^{iii}—Na1—Na1^{ix}  38.95 (3) 
F1—I1—O1^{iv}  89.87 (3)  O1^{v}—Na1—Na1^{x}  78.79 (4) 
F1^{ii}—I1—O1^{iv}  89.87 (3)  O1^{vi}—Na1—Na1^{x}  101.21 (4) 
O1^{iii}—I1—O1^{iv}  108.48 (7)  F1^{vii}—Na1—Na1^{x}  141.05 (3) 
O1^{v}—Na1—O1^{vi}  180.000 (17)  F1—Na1—Na1^{x}  38.95 (3) 
O1^{v}—Na1—F1^{vii}  92.23 (6)  F1^{viii}—Na1—Na1^{x}  38.95 (3) 
O1^{vi}—Na1—F1^{vii}  87.77 (6)  F1^{iii}—Na1—Na1^{x}  141.05 (3) 
O1^{v}—Na1—F1  87.77 (6)  Na1^{ix}—Na1—Na1^{x}  180.0 
O1^{vi}—Na1—F1  92.23 (6)  I1—O1—Na1^{xi}  139.35 (10) 
F1^{vii}—Na1—F1  180.00 (9)  I1—F1—Na1  128.21 (4) 
O1^{v}—Na1—F1^{viii}  87.77 (6)  I1—F1—Na1^{x}  128.21 (4) 
O1^{vi}—Na1—F1^{viii}  92.23 (6)  Na1—F1—Na1^{x}  102.11 (6) 
F1^{vii}—Na1—F1^{viii}  103.65 (7)   
Symmetry codes: (i) x, y, −z+1/2; (ii) −x+1, y, z; (iii) x, −y, −z; (iv) −x+1, −y, z+1/2; (v) x−1/2, y−1/2, z; (vi) −x+1/2, −y+1/2, −z; (vii) −x, −y, −z; (viii) −x, y, z; (ix) −x, −y, z−1/2; (x) −x, −y, z+1/2; (xi) x+1/2, y+1/2, z. 
Experimental details
Crystal data 
Chemical formula  NaIO_{2}F_{2} 
M_{r}  219.89 
Crystal system, space group  Orthorhombic, Cmcm 
Temperature (K)  293 
a, b, c (Å)  6.9287 (10), 7.2735 (13), 7.3503 (13) 
V (Å^{3})  370.42 (11) 
Z  4 
Radiation type  Mo Kα 
µ (mm^{−}^{1})  8.65 
Crystal size (mm)  0.10 × 0.10 × 0.02 

Data collection 
Diffractometer  Nonius KappaCCD diffractometer 
Absorption correction  Multiscan (SADABS; Bruker 2001) 
T_{min}, T_{max}  0.478, 0.846 
No. of measured, independent and observed [I > 2σ(I)] reflections  3335, 312, 303 
R_{int}  0.021 
(sin θ/λ)_{max} (Å^{−}^{1})  0.703 

Refinement 
R[F^{2} > 2σ(F^{2})], wR(F^{2}), S  0.010, 0.023, 0.75 
No. of reflections  312 
No. of parameters  22 
Δρ_{max}, Δρ_{min} (e Å^{−}^{3})  0.40, −0.57 
Selected bond lengths (Å) topI1—O1  1.7647 (17)  Na1—O1^{ii}  2.3124 (18) 
I1—F1  2.0040 (17)  Na1—F1  2.3627 (11) 
I1—O1^{i}  2.8185 (19)   
Symmetry codes: (i) x, −y, −z; (ii) x−1/2, y−1/2, z. 
Bond valences topAtom  Na1  I1  V_{ij} 
O1  0.253  1.903 + 0.110  2.27 
O1  0.253  1.903 + 0.110  2.27 
F1  2 × 0.157  0.718  1.07 
F1  2 × 0.157  0.718  1.07 
V_{ij}  1.14  5.46  
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The existence of sodium iodine(V) oxyfluoride, NaIO_{2}F_{2}, has been reported without structural characterization (Nikolaeva et al., 1988). As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, we report here the structure of NaIO_{2}F_{2}. Iodine(V) phases are expected to present some similarities with their tellurium(IV) homologues because of their almost identical ionic radius, in spite of their lower thermal stability.
In NaIO_{2}F_{2}, the Na^{+} atom is in a sixfold coordination, at the centre of a nearly regular NaO_{2}F_{4} octahedron (Fig. 1 and Table 1). Each Na^{+} atom is surrounded by four F atoms forming a square base and two apical O atoms.
The I^{V} cation is surrounded by four anions, namely two equatorial O atoms, and two axial F atoms at a longer distance (Table 1). Full O/F anionic order on the O1 and F1 sites is evidenced by bondvalence calculations (Brown, 1981) ? (Table 2). The corresponding polyhedron (Fig. 2) can be described as a trigonal bipyramid, IO_{2}F_{2}E, the fifth corner of which is occupied by the lone pair, E. The introduction of two weaker I1—O1 bonds turns the IO_{2}F_{2}E^{−} trigonal bipyramid into a pentagonal bipyramid, IO_{4}F_{2}E, but if the lone pair E is not included, the I^{5+} environment can also be considered as an IO_{4}F_{2}^{−} octahedron distorted by the repulsion of this lone pair on O1^{i} and O1^{iv} [symmetry codes: (i) x, −y, −z; (iv): −x + 1, −y, z + 1/2] (Fig. 2). This IO_{2}F_{2}^{−} polyhedron is essentially the same as those in KIO_{2}F_{2} (Abrahams & Bernstein, 1976) and [N(CH_{3})_{4}][IO_{2}F_{2}][HF_{2}] (Gerken et al., 2004). Weak I1—O1 bonds are also systematically present in these structures.
A comparison between I^{V} and Te^{IV} is very meaningful because their lone pairs have the same stereochemical activity (Glay et al., 1975). Some isolated polyhedra displaying such activity that can be compared with IO_{2}F_{2}^{−} include TeOF_{4}^{2−} in Cs_{2}TeOF_{4} (Jansen & Kessler, 2001), and IOF_{3} (Edwards & Taylor, 1974), TeF_{4} (Kniep et al., 1984) and IF_{5} (Burbank & Jones, 1974) in the corresponding phases.
Depending on the respective distribution of O and F anions in these structures, the classical umbrella effect resulting from the stereochemical activity of the lone pair E is more or less marked. In TeF_{4}, the axial angle F2—Te1—F4 is 161.31 (19)° and the equatorial angle F1—Te1—F3 is 87.59 (21)°. In IF_{5}, the average of the axial F—I1—F angles is 162.97 (63)°. In oxyfluorides, these angles increase, e.g. in IOF_{3} the axial F1—I1—F2 angle is 165.92 (11)° and the equatorial O1—I1—F3 angle is 102.02 (13)°; the average value of axial F—Te1—F angles in TeOF_{4}^{2−} is 177.55°. These last two polyhedra are the most similar to the IO_{2}F_{2}^{−} configuration, in which the axial angle F1—I1—F1^{iii} is 179.55 (10)° and the equatorial angle O1—I1—O1^{iii} is 103.72 (12)° [symmetry code: (iii) −x + 1, y, −z + 1/2]. Therefore, in these oxyfluorides the umbrella effect, characterized in a first approximation by axial angles lower than 180°, is partly or almost completely counterbalanced by the repulsion between the equatorial O anions and the axial F anions.
A projection along the [001] direction (Fig. 3) shows that the Na^{+} cations and IO_{2}F_{2}^{−} complex anions form a CsCllike lattice (Hyde & Andersson, 1989). The pseudocubic Na1 subcell is in fact tetragonal, with a = 5.02 Å and c = 3.68 Å, and its centre is occupied by an IO_{2}F_{2}^{−} complex anion which is responsible for the distortion.
The IO_{4}F_{2}E pentagonal bipyramids (or IO_{4}F_{2} distorted octahedra), incorporating two weak I1—O1 bonds, form linear [IO_{2}F_{2}^{−}]_{n} chains extending along the c axis through O—O edge sharing. Along this same axis, NaO_{2}F_{4} octahedra share F—F edges, also forming parallel linear chains alternating with [IO_{2}F_{2}^{−}]_{n} ones and shifted by c/4 (Fig. 4). Successive I1 and Na1 chains are connected along the [100] direction through F1 corners forming sheets. Along [010], these sheets are interconnected only via O1 corners of NaO_{2}F_{4} octahedra (Fig. 3). In this way, the structure can be described as a smooth threedimensional array of nearly perfect edgesharing NaO_{2}F_{4} octahedra and distorted IO_{4}F_{2} octahedra. From this point of view, the structure of NaIO_{2}F_{2} is derived from a rutile superstructure (Fig. 5) by a cationic I1/Na1 ordering in such a way that a = (a_{rutile})^{1/2}, b = (a_{rutile})^{1/2} [(b_{rutile})^{1/2} ?] and c = (c_{rutile})^{1/2}. This kind of structure is well adapted to cations presenting a stereochemically active electronic lone pair. For example, the structure of BiOF and PbFCl is described by Hyde & Andersson (1989) as being derived from the rutile type. The puckering of the anionic hexagonal closepacked planes, common in rutile phases and in TiO_{2} itself, accommodates in NaIO_{2}F_{2} the distortion resulting from the lonepair activity of I^{5+}.
In the MIO_{2}F_{2 }series, before the present work, only the KIO_{2}F_{2} structure was known. Described in the polar Pca2_{1} space group, it is also derived from CsCl, but the much greater size of K^{+} compared with Na^{+} leads to a complete separation of corrugated planes of IO_{2}F_{2}^{−} polyhedra, alternating with slightly twisted K^{+} square layers.
There is no evidence for noncentrosymmetry in NaIO_{2}F_{2}, as attempts to refine in a subgroup of Cmcm did not lead to a significantly better refinement. Moreover, no unusually large atomic displacement parameters for F and O are detected as in KIO_{2}F_{2}.
In conclusion, in NaIO_{2}F_{2}, as in KIO_{2}F_{2}, the IO_{2}F_{2}^{−} complex ion is present and seems to be a very stable unit. The crystal structure of NaIO_{2}F_{2}, in spite of some analogies with KIO_{2}F_{2}, is clearly different as a consequence of the difference in size between Na^{+} and K^{+}. If weak I1—O1 bonds are considered, the CsClderived stacking of isolated M^{+} and IO_{2}F_{2}^{−} ions is better described as a threedimensional array of octahedra derived from the TiO_{2} rutile type with cationic Na1/I1 longrange ordering. A parallel can also be established with the structure of FeTeO_{3}F (Laval et al., 2008), which is derived from the αPbO_{2} type (to which belongs TiO_{2} under high pressure) but with Fe/Te longrange ordering. It would be interesting to see if NaIO_{2}F_{2} under pressure presents the classical rutile → αPbO_{2} transition between these two main types of hexagonal closepacked anionic array (Hyde & Andersson, 1989).