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, NH4′ NPhH3′ or Hpy, but the monoalkylammonium salts [NRH3]2[CrBr4], where R = Me, Et, Prn, Bun, n-C5H11,n-C8H17′, or n-C12H25′[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 CrII 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 JBr > JCl. 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.