Transition metal oxalate complex

Transition metal oxalate complexes are coordination complexes with oxalate (C₂O₄²⁻) ligands. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C₂O₄^2-) is a kind of dicarboxylate ligand.^[1] As a small, symmetrical dinegative ion, oxalate commonly forms five-membered MO₂C₂ chelate rings. Mixed ligand complexes are known, e.g., [Co(C₂O₄)(NH₃)₄]^κ+.^[2]

Structures of some metal oxalate complexes

• 
  [Cr₂(ox)₅]^4-.
• 
  Potassium ferrioxalate (K₃[Fe(C₂O₄)₃]·3H₂O)
• 
  Oxaliplatin, an anticancer drug
• 
  [Zr(ox)₄]^4-

Examples

Homoleptic complexes

Homoleptic oxalato complexes are common, e.g., those with the formula [M(κ²-C₂O₄)₃]^n-: M = V(III), Mn(III),^[3] Cr(III), Tc(IV), Fe(III), Ru(III), Co(III), Rh(III), Ir(III). These anions are chiral (D₃ symmetry), and some have been resolved into their component enantiomers.^[4] Some early metals form tetrakis complexes of the type [M(κ²-C₂O₄)₄]^n- M = Nb(V),^[5] Zr(IV),^[6] Hf(IV),^[7] Ta(V),^[8]

The Δ and Λ enantiomorphs of [Fe(C₂O₄)₃]³⁻ have been separated.

Structure of hydrated ferric oxalate, a coordination polymer.^[9] Color code: red=O, white = H, blue = Fe, gray = C.

Oxalate is often a bridging ligand forming bi- and polynuclear complexes with (κ²,κ'²-C₂O₄)M₂ cores. Illustrative binuclear complexes are [M₂(C₂O₄)₅]^2- M = Fe(II)^[10] and Cr(III)^[11]

Mixed ligand complexes

Whereas homoleptic complexes are easier to describe, far more abundant are complexes with oxalate and other ligands. Many metals form polynuclear complexes with oxalate and water.^[12] In ferric oxalate, Fe₂(C₂O₄)₃·4H₂O, one oxalate is bonded through all four oxygen atoms and another oxalate binds through only two oxygen atoms, in both cases bridging. "Durrant's salt" contains the anionic complex [Co(C₂O₄)₂(mu−⁻OH)]4−2.^[13]

Reactions and applications

Potassium ferrioxalate crystals.

Metal oxalate complexes are photoactive, degrading with loss of carbon dioxide. This reaction is the basis of the technique called actinometry. Ferrioxalate undergoes photoreduction. The iron centre is reduced (gains an electron) from the +3 to the +2 oxidation state, while an oxalate ion is oxidised to carbon dioxide:

2 [Fe(C
₂O
₄)
₃]³⁻ + hν → 2 [Fe(C
₂O
₄)
₂]²⁻ + 2 CO
₂ + C
₂O²⁻
₄

The redox reaction has been used to access unusual complexes. UV-irradiation of Pt(C₂O₄)(PPh₃)₂ gives derivatives of Pt⁰(PPh₃)₂.

Metal oxalates with the stoichiometry 1:1 are often insoluble. This fact provides a way to separate metal ions from solutions, including extract of ores. Combustion of metal oxalates gives metal oxides.^[14]

Natural occurrence

The minerals moolooite and antipinite are examples of naturally occurring copper oxalates. They arise from the weathering of other copper ores. A few other oxalate-containing minerals are known.^[15]

See also

• Transition metal carboxylate complex
• Oxalatonickelate