The polar [WO2F4](2-) anion in the solid state

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 co ntaining [WO2F4](2-) must be understood as a first step toward engineering a polar SHG material with this anion. The strategy of crystallizing the [WO 2F4](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 ther mal ellipsoids in disordered [WO2F4](2-) compounds gives insight into disor der mechanisms and suggests strategies for eliminating orientational disord er. Crystal data: for [pyH](2)-[Cu(py)(4)(WO2F4)(2)], tetragonal, space gro up I4(1)22 (No. 98), with a 11.339(2) Angstrom, c = 30.489(6) Angstrom, and Z = 4; for Cu(pyz)(2)WO2F4.(pyz)(H2O) polymorph I, tetragonal, space group I4/mcm (No. 140), with a = 9.745(1) Angstrom, c = 16.565(3) Angstrom, and Z = 4; for Cu(pyz)(2)WO2F4.(pyz)(H2O) polymorph II, tetragonal, space group P4/mmm (No. 123), with a = 6.886(3) Angstrom, c = 8.300(2) Angstrom, and Z = 1.