Chromium(II) chemistry. Part 14. Ferromagnetic and antiferomagnetic tetrabromochromates(II)

Abstract

The dihydrated chromium(II) complex bromides M₂[CrBr₄(OH₂)₂], where M = Cs, Rb, NH₄, or Hpy (py = pyridine), are magnetically dilute, high-spin compounds. The ammonium salt is isomorphous with the analogous copper(II) bromide which is known to contain trans-[CuBr₄(OH₂)₂]^2– anions with two short and two long Cu–Br bonds. The piperazinium salts [H₂pipz]₂[CrBr₆]·2H₂O and [H₂pipz]₂[CrBr₆], and the guanidinium salt [C(NH₂)₃]₂[CrBr₄]·2MeCO₂H, are also magnetically dilute. Their reflectance spectra are as expected for tetragonally distorted octahedral anions. Thermal dehydration gives the compounds M₂[CrBr₄], where M = Cs, Rb, NH_4′ NPhH_3′ or Hpy, but the monoalkylammonium salts [NRH₃]₂[CrBr₄], where R = Me, Et, Prⁿ, Buⁿ, n-C₅H₁₁,n-C₈H_17′, or n-C₁₂H_25′[NMe₂H₂]₂[CrBr₄], and [C(NH₂)₃]₂[CrBr₄]·2MeCo₂H have been crystallised from glacial acetic acid, and [NEt₄]₂[CrBr₄], from mixed organic solvents. Ferromagnetic behaviour is exhibited by Cs₂[CrBr₄] and the monoalkylammonium salts, but the other tetrabromochromates(II) are antiferromagnetic. The reflectance spectra indicate tetragonal six-co-ordination of Cr^II and hence polymeric structures. Like the analogous tetrachlorochromates(II), the magnetic data for the ferromagnetic tetrabromochromates from liquid-nitrogen to room temperature can be reproduced by the high-temperature series-expansion formula for a sheet ferromagnet with S= 2, and J_Br > J_Cl. The reflectance spectra contain very sharp bands at ca. 15 600 and 18 400 cm^–1 assigned to spin-forbidden transitions intensified by magnetic coupling. The magnetic behaviour of the antiferromagnetic complexes is reproduced by the high-temperature series-expansion formula with appropriate changes of sign. The stretching vibrations of the short Cr–Br bonds occur near 250 cm^–1.