IronII

Iron(II), also called ferrous, refers to the iron ion in the +2 oxidation state, Fe2+. It is one of the common oxidation states of iron, the other being iron(III) (Fe3+). In aqueous solution Fe2+ forms pale green to blue-green ions; in solid salts the color varies with the counterion and coordination environment.

The electronic configuration of Fe2+ is [Ar]3d6. In coordination complexes, Fe2+ can be high-spin (typical with weak-field ligands such as water) or low-spin (with strong-field ligands such as cyanide). High-spin Fe2+ has four unpaired electrons, while low-spin Fe2+ can have fewer or zero unpaired electrons. This spin state affects magnetic and spectroscopic properties.

Ferrous compounds include ferrous sulfate (FeSO4), ferrous chloride (FeCl2), and ferrous carbonate (FeCO3). Fe2+ ions readily oxidize to ferric (Fe3+) in the presence of oxygen or oxidizing agents, a tendency that often leads to the formation of iron(III) oxides or hydroxides. As a result, Fe2+ is typically a mild reducing agent in chemical reactions.

Biologically, Fe2+ centers appear in heme proteins (such as deoxyhemoglobin and myoglobin) and in iron-sulfur proteins like ferredoxins, where iron cycles between +2 and +3 oxidation states to facilitate electron transfer.

In geology and environmental contexts, iron(II) minerals and dissolved Fe2+ play roles in redox chemistry and nutrient cycling. Handling practices consider the propensity of Fe2+ to oxidize in air, which can influence stability and color.