Chromium(II) chemistry. Part 12. Further examples of ferromagnetic chlorochromates(II)

Abstract

The chlorochromates(II)[NMeH₃]₂[CrCl₄], [NEtH₃]₂[CrCl₄], [H₂en][CrCl₄], [H₃dien][CrCl₄]Cl, and [H₄trien][CrCl₄]Cl₂(en, dien, and trien are ethylenediamine, 3-azapentane-1,5-diamine, and 3,6-diazaoctane-l,8-diamine respectively) are ferromagnetic whereas [NMe₂H₂]₂[CrCl₄] and [NMe₂H₂][CrCl₃] are antiferromagnetic. The ferromagnetic complexes are isomorphous with the corresponding copper(II) complexes. These are known to contain two-dimensional networks of planar [CuCl₄]^2– units bridged by chloride ions of adjacent units so that the metal ions occupy tetragonally elongated octahedra. The magnetic behaviour of the ferromagnetic chromium(II) complexes from room to liquid-nitrogen temperature is reasonably represented by the high-temperature series-expansion formula for a sheet ferromagnet. The reflectance spectra of all the complexes are characteristic of six-co-ordinate Cr^II, and, in the case of the ferromagnetic complexes, contain very sharp bands at ca. 15 800 and 18 700 cm^–1 assigned to spin-forbidden bands intensified by magnetic coupling. Absorption bands in the far-i.r. spectra near 300 cm^–1 are assigned to Cr–Cl stretching vibrations within the planar [CrCl₄]^2– units.