Ferric Ammonium Citrate – What's in It?

Abstract Ferric ammonium citrate (FAC) is administered to humans in drugs or food supplements, but its exact composition and structural features are not known. We report that the major component in commercial FAC‐brown and FAC‐green is a trinuclear ferric citrate complex, namely [Fe 3 (cit) 4 H] 6– ( 1 ). Complex 1 comprises a dinuclear subunit in which two ferric ions with a 3.122(1) Å separation are bridged by two alkoxido oxygen atoms from two citrate ligands. Two other alkoxido oxygen atoms form a bridge to a third ferric ion completing an Fe 3 O 4 inner core. The compound [Cr(urea) 6 ] 2 ·1· 14.5H 2 O exhibits paramagnetic behavior that corresponds to three magnetically independent high‐spin (HS) d 5 and two d 3 centers. In addition to complex 1 , FAC‐green also contains a dinuclear ferric citrate complex, namely [Fe 2 (Hcit) 3 ] 3– , in an approximate ratio of 1:1.