Second-sphere coordination in anion binding: sodium hexaamminecobalt(III) tetrakis(4-fluorobenzoate) monohydrate.

In sodium hexaamminecobalt(III) tetrakis(4-fluorobenzoate) monohydrate, Na[Co(NH3)6](C7H4FO2)4.H2O, determined at 180 K, [Co(NH3)6]3+ cations lie on centres of inversion and form layers in which their C4 axes lie perpendicular to the layer planes. 4-Fluorobenzoate anions lie on twofold axes and general positions and adopt near-planar geometries. Na+ cations and water molecules lie on twofold axes, forming [NaO5] square pyramids that lie between the [Co(NH3)6]3+ cations. The second-sphere interactions between [Co(NH3)6]3+ cations and 4-fluorobenzoate anions comprise edge-to-face and vertex-to-face arrangements. The structure is closely comparable with that of the benzoic acid salt, demonstrating that fluorination of the anion in the para position has no significant influence on the second-sphere interactions and minimal influence on the gross crystal structure.