Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K₃[Fe(C₂O₄)₃], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate

Oxalate

Oxalate , is the dianion with formula C2O42− also written 22−. Either name is often used for derivatives, such as disodium oxalate, 2C2O42−, or an ester of oxalic acid Oxalate (IUPAC: ethanedioate), is the dianion with formula C2O42− also written (COO)22−. Either...

 ions are bound to an iron center. Potassium acts as a counterion

Counterion

A counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.In a charged transition metal complex, a simple A counterion is the ion that accompanies an ionic species in...

, balancing the -3 charge of the complex. Crystals of the trihydrated form of the complex, K₃[Fe(C₂O₄)₃]·3H₂O, are emerald green in color.