Salts of the tris(isonitrosomalonamido)ferrate(II) ion: Crystal structures and anomalous magnetism of the potassium and caesium salts of the tris(isonitrosomalonamido)ferrate(II) anion

Complexes of the type M[FeL₃]·xH₂O [M⁺ = alkali-metal cation, L^- = NH₂·COC(NO)-·CO·NH₂] have been prepared. The sodium and rubidium salts are diamagnetic; the potassium and caesium salts are paramagnetic, their room temperature magnetic moments being 2.1₆ and 2.6₆ B.M. respectively with only a slight temperature dependence. Mössbauer spectra are indicative of the presence of both singlet and quintet states. The crystal structure of the potassium salt, K[FeL₃]·2H₂O, has been determined from diffractometer data and refined by least squares to R 0.040 for 3 981 observed reflections. Crystals are monoclinic, space group C2/c, a = 29.89(1), b = 9.780(2), c = 13.980(3) Å, β = 106.68(2)°, Z = 8. The cell contents comprise infinite zig-zag chains of K⁺ cations and [FeL₃]^- anions, the ligands co-ordinating through one of the amide oxygens and the nitrosyl nitrogen in a fac-configuration, interspersed by potassium cations, the co-ordination sphere of the latter being the three nitrosyl oxygens from one anion, and the three remaining amide oxygens from the next; the chains are parallel to c; mean distances: Fe-N 1.88₂, Fe-O 1.97₂ Å. The crystal structure of Cs[FeL₃]·2H₂O has also been determined by X-ray diffraction and refined by least squares to R 0.12 for 5 126 observed reflections. Crystals are monoclinic, space group Aa, a = 33.312(6), b = 24.792(5), c = 13.323(2) Å, β = 91.55(1)°. Z = 20 (for Cs[FeL₃]). As with K[FeL₃]·2H₂O, the cell contents comprise parallel independent chains of [FeL₃]^-anions alternating with Cs cations, the chains being almost linear in this case; one of the chains is interrupted by a region of considerable disorder which adversely affects the accuracy of determination of the metal-atom geometries. Neither structure determination is helpful in understanding the anomalous magnetism of these two complexes.