The Ordered [WO₂F₄]²⁻ Anion

Crystallographic order of the [WO2F4]2- anion has been achieved in the new compound [HNC6H6OH][Cu(NC5H5)4(WO2F4)2]. This salt with two cations, tetrakis(pyridine)copper(II) [Cu(NC5H5)4]2+ and 5-hydroxy-2-methylpyridinium [HNC6H6OH]+, results in three separate anion−cation contacts and eliminates the disorder common to the [WO2F4]2- anion in the solid state. Two of those interactions are hydrogen bonds formed with [HNC6H6OH]+, through the protonated amine and the hydroxyl group. The third contact is formed with [Cu(py)4]2+. These results and observations explain the trans-directing nature of the [WO2F4]2- anion and the distinctly different cis-directing nature of the group 6 analogue [MoO2F4]2-.