Iron(III)

In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe³⁺.

The adjective ferric or the prefix ferri- is often used to specify such compounds, as in ferric chloride for iron(III) chloride (FeCl₃). The adjective ferrous is used instead for iron(II) salts, containing the cation Fe²⁺. The word ferric is derived from the Latin word ferrum, meaning "iron".

Iron(III) metal centres also occur in coordination complexes, such as in the anion ferrioxalate, [Fe(C₂O₄)₃]³⁻, where three bidentate oxalate ions surrounding the metal centre; or, in organometallic compounds, such as the ferrocenium cation [Fe(C₂H₅)₂]⁺, where two cyclopentadienyl anions are bound to the Fe^III centre.

Iron is almost always encountered in the oxidation states 0 (as in the metal), +2, or +3. Iron(III) is usually the most stable form in air, as illustrated by the pervasiveness of rust, an insoluble iron(III)-containing material.