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 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 14₁22 (No. 98), with a = 11.339(2) ̊, c - 30.489(6) ̊, and Z = 4; for Cu(pyz)₂WO₂F4·(pyz)(H₂O) polymorph I, tetragonal, space group 14/mcm (No. 140), with a = 9.745(1) ̊, c = 16.565(3) ̊, and Z = 4; for Cu(pyz)₂WO₂F₄·(pyz)(H₂O) polymorph II, tetragonal, space group P4/mmm (No. 123), with a = 6.886(3) ̊, c = 8.300(2) ̊, and Z = 1.