Salts of the tris(violurato)ferrate(II) ion: crystal structureof ammonium tris(violurato)ferrate(II) hydrate

Salts of the type M⁺[FeV₃]^-·xH₂O have been prepared (where M⁺ = alkali-metal cation, V^- = violurate) and found to be diamagnetic. The crystal structure of the title compound [NH₄][FeV₃]·ca. 4.5H₂O has been determined by X-ray diffraction and refined by least squares to R 0.07 for 3 868 observed reflections. Crystals are triclinic, space group P1, a = 16.242(3), b = 15.690(3), c = 9.848(2) Å, α = 90.95(2), β = 106.99(2), γ = 74.20(2)°, Z = 4. There are two independent cations and anions in the asymmetric unit; the geometry of the anion co-ordinated by three bidentate violurate ligands (V^- = C₄O₄N₃H₂^-) is fac (means: Fe-O 1.98₃, Fe-N 1.87₇ Å), and closely resembles that found in ferroverdin. The accuracy of the structure is inhibited by serious disorder among the solvent molecules in the lattice.