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

Abstract

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 2.1 ₆ 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 1.97₂Å.

The crystal structure of Cs[FeL₃]·2H₂O has also been determined byX-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.