FeCl4

FeCl4 typically refers to the tetrachloroferrate(III) anion, [FeCl4]−, and to salts that contain this unit, such as potassium tetrachloroferrate (KFeCl4), sodium tetrachloroferrate (NaFeCl4), and analogous rubidium or cesium salts (RbFeCl4, CsFeCl4). A stable neutral FeCl4 molecule is not established under ordinary conditions; iron in the +4 oxidation state would have to bear four chloride ligands in a neutral complex, a species that is not commonly observed in practice.

Structure and oxidation state: In the prevalent anion [FeCl4]−, iron is in the +3 oxidation state and the coordination geometry around Fe is tetrahedral. The d5 configuration in this high-spin tetrahedral field gives rise to paramagnetism, with a spin-only magnetic moment near 5.9 Bohr magnetons.

Solutions and stability: The tetrachloroferrate(III) anion is typically formed and observed in strongly acidic, chloride-rich environments. In aqueous media outside such conditions, hydrolysis, oxidation, or conversion to other iron–chloride species can occur, so stability is largely dependent on the surrounding ligands and counterions. In solid form, the anion pairs with alkali metals to produce a family of salts that crystallize in various structures and can exhibit color dependent on hydration and crystal lattice effects.

Applications and context: FeCl4− salts are of interest in inorganic synthesis and structural chemistry as a convenient Tetrachloroferrate(III) unit for assembling iron(III) tetrahalometalate motifs. They serve as reagents for preparing iron coordination compounds and studying high-spin Fe(III) chemistry in chloride environments.