ОКСАЛАТ НАТРИЯ ЖЕЛЕЗА

Sodium ferri oxalate, also known as sodium oxalatoferrate, is a chemical compound with the formula Na₃[Fe(C₂O₄)₃], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions act as ligands bound to an iron centre. Sodium acts as a counterion, balancing the -3 charge of the complex. Crystals of the hydrated form of the complex, Na₃[Fe(C₂O₄)_x].xH₂O, are lime green in colour. In solution the complex dissociates to give the ferrioxalate anion, [Fe(C₂O₄)₃]^3-, which appears a deep apple green in colour.

Emerald-green crystals, decomposed by heat or light. protect from light. Soluble in water and alcohol.

The bonds to the iron atom are dative covalent bonds where the ligands, (oxalate ions, blue), donate a lone pair into the empty p and d orbitals of the transition metal (iron, red), atom. The three oxalate ions donate 12 electrons in all and Fe-III has three electrons in the d orbitals leaving 13 empty places in the remaining d and p orbitals.

The ferri oxalate complex demonstrates optical activity since there are two non-superimposable stereoisomers of the complex. This is described in more detail under potassium ferrioxalate. Theoretically the two stereoisomers could be separated by crystallization of a diastereomeric salt of the optically inactive racemic mixture of ferrioxalate ions with an optically active cation, such as methylethylpropylammonium ion which is one pure enantomer. Thus methylethylpropylammonium ferrioxalate should crystallize out to produce crystals which are non superimposable mirror images. These would be Λ-methyl ethyl propyl ammonium Λ-ferri oxalate and Λ- methyl ethyl propyl ammonium Δ-ferri oxalate.

Photography, blueprinting.

The crystals pictured were synthesised by mixing solutions of sodium oxalate and ferric oxalate and waiting a few hours for the brown colour of the ferric oxalate to be replaced with the green colour of the complex anion. This complex is relatively inert and the equilibrium is attained only slowly at room temperature. The ferric oxalate was made by dissolving rust in oxalic acid and filtering off any residual insolubles. The solution was evaporated at just below boiling until small crystals appeared on the bottom indicating the solution was then hot and saturated. The solution was allowed to cool in a beaker sitting on a large aluminium block. The thermal mass of the block allowed sufficiently slow cooling over night to produce crystals a few milimetres long. These larger crystals are pictured at the upper left.
Fe₂(C₂O₄)₃ + 3 Na₂(C₂O₄) → 2 Na₃[Fe(C₂O₄)₃]
Stoichiometry was not worried about and an excess of sodium oxalate was added, this is a lot less soluble in hot water than the ferrioxalate and crystallizes out first. The intensity of the green colour was used as a guide to concentration of the solution with respect to the complex. A few drops of 100 vol hydrogen peroxide were periodically added during the evaporation to maintain the iron in the III oxidation state and any insoluble ferrous oxalate was removed if it precipitated out.