Ferrate(VI)

Ferrate(VI)

200px Solutions of ferrate (left) and permanganate (right)
Names
IUPAC name Ferrate(VI)
Systematic IUPAC name Tetraoxoironbis(olate)
Other names Ferrate(2-)
Identifiers
ChEBI	CHEBI:30992 Y
ChemSpider	21865127 Y
Jmol 3D model	Interactive image Interactive image
PubChem	25000034
InChI InChI=1S/Fe.4O/q;;;2*-1 Y Key: XGBDPAYTQGQHEW-UHFFFAOYSA-N Y InChI=1/Fe.4O/q;;;2*-1/rFeO4/c2-1(3,4)5/q-2 Key: XGBDPAYTQGQHEW-WTZHFVRHAI
SMILES [O-][Fe]([O-])(=O)=O [O-][Fe](=O)(=O)[O-]
Properties
Chemical formula	FeO42−
Molar mass	119.843 g mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Ferrate(VI) is the inorganic anion with the chemical formula [FeO₄]²⁻. It is photosensitive, contributes a pale violet colour to compounds and solutions containing it and is one of the strongest water-stable oxidising species known. Although it is classified as a weak base, concentrated solutions containing ferrate(VI) are corrosive and attack the skin and are only stable at high pH.

Nomenclature

The term ferrate is normally used to mean ferrate(VI), although it can refer to other iron-containing anions, many of which are more commonly encountered than salts of [FeO₄]²⁻. These include the highly reduced species disodium tetracarbonylferrate Na₂[Fe(CO)₄] and salts of the iron(III) complex tetrachloroferrate [FeCl₄]⁻. Although rarely studied, ferrate(V) [FeO₄]³⁻ and ferrate(IV) [FeO₄]⁴⁻ oxyanions of iron also exist. These too are called ferrates.

Synthesis

Ferrate(VI) salts are formed by oxidizing iron in an aqueous medium with strong oxidizing agents under alkaline conditions, or in the solid state by heating a mixture of iron filings and powdered potassium nitrate.

For example, ferrates are produced by heating iron(III) hydroxide with sodium hypochlorite in alkaline solution:

2 Fe(OH)
₃ + 3 OCl⁻
+ 4 OH⁻
→ 2 [FeO
₄]²⁻
+ 5 H
₂O + 3 Cl⁻

The anion is typically precipitated as the barium(II) salt, forming barium ferrate.

Properties

The ferrate(VI) anion is unstable at neutral or acidic pH values, decomposing to iron(III):

[FeO
₄]²⁻
+ 3 ^-
_e + 8 H⁺
⇌ Fe³⁺
+ 4 H
₂O

The reduction goes through intermediate species in which iron has oxidation states +5 and +4. These anions are even more reactive than ferrate(VI). In alkaline conditions ferrates are more stable, lasting for about 8 to 9 hours at pH 8 or 9.

Aqueous solutions of ferrates are pink when dilute, and deep red or purple at higher concentrations. The ferrate ion is a stronger oxidizing agent than permanganate, and will oxidize chromium(III) to dichromate, and ammonia to molecular nitrogen.

Ferrates are excellent disinfectants, and are capable of removing and destroying viruses.

The ferrate(VI) ion has two unpaired electrons, and is thus paramagnetic. It has a tetrahedral molecular geometry.

See also

• Potassium ferrate