The Polar [WO2F4] Anion in the Solid State

The cis-[WO2F4] anion is inherently acentric and contains polarizable WdO bonds, making it a good candidate for second-harmonic generation (SHG). However, the orientational disorder commonly found in crystal structures containing [WO2F4] must be understood as a first step toward engineering a polar SHG material with this anion. The strategy of crystallizing the [WO2F4] anion with two different cations, which was successful in ordering the related [NbOF5] and cis-[MoO2F4] anions, is able to order one of the two WdO bonds. Close inspection of bond lengths and anisotropic thermal ellipsoids in disordered [WO2F4] 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.