The Polar [WO₂F₄]^2- Anion in the Solid State

Abstract

The cis-[WO2F4]2- anion is inherently acentric and contains polarizable WO bonds, making it a good candidate for second-harmonic generation (SHG). However, the orientational disorder commonly found in crystal structures containing [WO2F4]2- must be understood as a first step toward engineering a polar SHG material with this anion. The strategy of crystallizing the [WO2F4]2- anion with two different cations, which was successful in ordering the related [NbOF5]2- and cis-[MoO2F4]2- anions, is able to order one of the two WO bonds. Close inspection of bond lengths and anisotropic thermal ellipsoids in disordered [WO2F4]2- compounds gives insight into disorder mechanisms and suggests strategies for eliminating orientational disorder. Crystal data: for [pyH]2[Cu(py)4(WO2F4)2], tetragonal, space group I4122 (No. 98), with a = 11.339(2) Å, c = 30.489(6) Å, and Z = 4; for Cu(pyz)2WO2F4·(pyz)(H2O) polymorph I, tetragonal, space group I4/mcm (No. 140), with a = 9.745(1) Å, c = 16.565(3) Å, and Z = 4; for Cu(pyz)2WO2F4·(pyz)(H2O) polymorph II, tetragonal, space group P4/mmm (No. 123), with a = 6.886(3) Å, c = 8.300(2) Å, and Z = 1.

New Layered Oxide-Fluoride Perovskites: KNaNbOF5 and KNaMO2F4 (M = Mo6+, W6+)

“…Despite the partial ordering of the oxide and fluoride, the A-and B-site cations are ordered. The partial ordering of the anionic unit has been reported previously, where Mo/W distorts towards the corner as opposed to the edge [22,23]. The partial ordering of the oxide and fluoride is due to the symmetric environment around the equatorial positions.…”

“…The B-site cations, Na + and M 6+ , form corner-sharing octahedra (no tilt, a 0 a 0 a 0 ). The M 6+ ions form one short M=O and one long M-F bond trans to the oxide (Table 1) and, as a result, distort toward the corners of the octahedra as opposed to the edges [22,23]. 2-anionic unit [20,23].…”

Hydrolysis of ammonium oxofluorotungstates: A 19F, 17O and 183W NMR study

“…It is known that the O atoms and F t atoms in [WO 2 F 4 ] 2À are the most nucleophilic centers [17]. Most probably, a water molecule attacks a F t atom with the formation of F t Á Á ÁH-O-HÁ Á ÁF t hydrogen bonds followed by HF elimination and the formation of the Obridged dimer.…”

“…It should be noted that assignments of the high-and low-frequency triplets are reversed in the cases of W and Mo relative to those of Os, Tc, and Re. In the latter cases, 187 Os, 99 Tc and 17 In the present work, aqueous solutions of (NH 4 ) 2 WO 2 F 4 and (NH 4 ) 3 WO 3 F 3 were investigated by 19 F, 17 O and 183 W NMR spectroscopy.…”

Noncentrosymmetric Inorganic Oxide Materials: Synthetic Strategies and Characterisation Techniques

“…[54] The material exhibits a three-dimensional crystal structure consisting of corner-shared BO 4 Figure 1.5). [48,[69][70][71][72][73][74][75][76][77][78] Similar to the 'pure' d 0 oxides systems discussed earlier, the metal cation in the centre of the oxyfluoride octahedra spontaneously displaces toward a corner ( This spontaneous distortion is inherent to the oxyfluoride anion and is a result of metal-dp-oxygen-pp orbital interactions. For example, in anions such as [NbOF 5 ] 2À , the Nb 5þ cation distorts toward the oxide ligand, resulting in a short Nb-O bond and a long, trans Nb-F bond.…”