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]

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.

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]

With only oxalate and water, many metals form polynuclear complexes.^[12] In such materials, oxalate typically functions as a κ²-bidentate ligand, as illustrated by ferrioxalate, [Fe(C₂O₄)₃]³⁻. The Δ and Λ enantiomorphs of this trianion have been separated. 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. 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.

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.^[13]

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.^[14]

• Transition metal carboxylate complex
• Oxalatonickelate