Iron(II) bromide

Iron(II) bromide refers to inorganic compounds with the chemical formula FeBr₂(H₂O)_x. The anhydrous compound (x = 0) is a yellow or brownish-colored paramagnetic solid. The tetrahydrate is also known, all being pale colored solids. They are common precursor to other iron compounds.

Like most metal halides, FeBr₂ adopts a polymeric structure consisting of isolated metal centers cross-linked with halides. It crystallizes with the CdI₂ structure, featuring close-packed layers of bromide ions, between which are located Fe(II) ions in octahedral holes. The packing of the halides is slightly different from that for FeCl₂, which adopts the CdCl₂ motif. The tetrahydrates FeX₂(H₂O)₄ (X = Cl, Br) have similar structures, with octahedral metal centers and mutually trans halides.

FeBr₂ is synthesized using a methanol solution of concentrated hydrobromic acid and iron powder. It adds the methanol solvate [Fe(MeOH)₆]Br₂ together with hydrogen gas. Heating the methanol complex in a vacuum gives pure FeBr₂.

FeBr₂ reacts with two equivalents of tetraethylammonium bromide to give [(C₂H₅)₄N]₂FeBr₄. FeBr₂ reacts with bromide and bromine to form the intensely colored, mixed-valence species [FeBr₃Br₉]⁻.

FeBr₂ possesses a strong metamagnetism at 4.2 K and has long been studied as a prototypical metamagnetic compound.