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'''Transition metal oxalate complexes''' are [[coordination complex]]es with [[oxalate]] (C<sub>2</sub>O<sub>4</sub><sup>2−</sup>) [[ligand]]s. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C<sub>2</sub>O<sub>4</sub><sup>2-</sup>) is a kind of dicarboxylate ligand.<ref>{{cite journal |doi=10.1021/cr60211a001|title=The Chemistry of the Metal Oxalato Complexes|year=1961|last1=Krishnamurty|first1=Kotra V.|last2=Harris|first2=Gordon M.|journal=Chemical Reviews|volume=61|issue=3|pages=213–246}}</ref>  As a small, symmetrical dinegative ion, oxalate commonly forms five-membered MO<sub>2</sub>C<sub>2</sub> chelate rings. Mixed ligand complexes are known, e.g., [Co(C<sub>2</sub>O<sub>4</sub>)(NH<sub>3</sub>)<sub>4</sub>]<sup>κ+</sup>.<ref>{{cite journal |doi=10.1007/BF00674267|title=The phenomenon of conglomerate crystallization. XIX. Clavic dissymmetry in coordination compounds. XVII|year=1990|last1=Bernal|first1=Ivan|last2=Cetrullo|first2=James|journal=Structural Chemistry|volume=1|issue=2–3|pages=235–243|s2cid=96021604}}</ref>

{{Gallery
|title = Structures of some metal oxalate complexes
|align = center
|File:Structure of Cr2(ox)5 tetraanion (POWVOM).png
| [Cr<sub>2</sub>(ox)<sub>5</sub>]<sup>4-</sup>.
|File:Potassium-trioxalatoferrate(III)-2D.png
| [[Potassium ferrioxalate]] ({{chem2|K3[Fe(C2O4)3]*3H2O}})
|File:Oxaliplatin-2D-skeletal.svg
| [[Oxaliplatin]], an anticancer drug
|File:Zr(ox)4 tetraanion.svg
| [Zr(ox)<sub>4</sub>]<sup>4-</sup>
}}

==Homoleptic complexes==
Homoleptic oxalato complexes are common, e.g., those with the formula [M(κ<sup>2</sup>-C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]<sup>n-</sup>: M = V(III), Mn(III),<ref>{{cite journal |doi=10.1107/S0567740880007558|title=Structure of potassium tris(oxalato)manganate(III) trihydrate|year=1980|last1=Lis|first1=T.|last2=Matuszewski|first2=J.|journal=Acta Crystallographica Section B|volume=36|issue=8|pages=1938–1940}}</ref> Cr(III), Tc(IV), Fe(III), Ru(III), Co(III), Rh(III), Ir(III). These anions are chiral (D<sub>3</sub> symmetry), and some have been resolved into their component enantiomers.<ref>{{cite book |doi=10.1002/9780470132395.ch55|title=Resolution of the Trioxalatocobaltate(III) Ion|series=Inorganic Syntheses|year=1966|last1=Kauffman|first1=George B.|last2=Takahashi|first2=Lloyd T.|last3=Sugisaka|first3=Nobuyuki|volume=8|pages=207–211|isbn=9780470132395}}</ref>  Some early metals form tetrakis complexes of the type [M(κ<sup>2</sup>-C<sub>2</sub>O<sub>4</sub>)<sub>4</sub>]<sup>n-</sup> M = Nb(V),<ref>{{cite journal |doi=10.1021/ic00264a035|title=Variable Stereochemistry of the Eight-Coordinate Tetrakis(oxalato)niobate(IV), Nb(C<sub>2</sub>O<sub>4</sub>)<sub>4</sub><sup>4-</sup>|year=1987|last1=Cotton|first1=F. A.|last2=Diebold|first2=Michael P.|last3=Roth|first3=W. J.|journal=Inorganic Chemistry|volume=26|issue=17|pages=2889–2893}}</ref> Zr(IV),<ref>{{cite journal |doi=10.1107/S1600536805033829|title=Bis(4,4′-bipyridinium) Tetrakis(oxalato-κ2O,O′)zirconate(IV)|year=2005|last1=Fu|first1=Yun-Long|last2=Ren|first2=Jia-Lin|last3=Xu|first3=Zhi-Wei|last4=Ng|first4=Seik Weng|journal=Acta Crystallographica Section E|volume=61|issue=11|pages=m2397–m2399}}</ref> Hf(IV),<ref>{{cite journal |doi=10.1107/S0567740877010395|title=Structure cristalline du tétrakisoxalatohafniate de Potassium Pentahydraté &#91;K4Hf(C2O4)4.5H2O&#93;|year=1977|last1=Tranqui|first1=D.|last2=Boyer|first2=P.|last3=Laugier|first3=J.|last4=Vulliet|first4=P.|journal=Acta Crystallographica Section B|volume=33|issue=10|pages=3126–3133}}</ref> Ta(V),<ref>{{cite journal |doi=10.1007/s11224-009-9494-0|title=&#91;NH<sub>4</sub>&#93;&#91;(CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub>&#93;2&#91;Ta(C<sub>2</sub>O<sub>4</sub>)<sub>4</sub>&#93;·2H<sub>2</sub>O: The First (Oxalato)tantalate(V) Complex Structurally Characterized|year= 2009|last1=Perić|first1=Berislav|last2=Brničević|first2=Nevenka|last3=Jurić|first3=Marijana|last4=Planinić|first4=Pavica|last5=Matković-Čalogović|first5=Dubravka|journal=Structural Chemistry|volume=20|issue=5|pages=933–941|s2cid=96838371}}</ref>

The Δ and Λ enantiomorphs of {{chem2|[Fe(C2O4)3](3-)}} have been separated.


[[File:Fe2(ox)3hydrate.jpg|thumb|Structure of hydrated ferric oxalate, a [[coordination polymer]].<ref name=ahou>{{cite journal | last1 = Ahouari | first1 = Hania | last2 = Rousse | first2 = Gwenaëlle | last3 = Rodríguez-Carvajal | first3 = Juan | last4 = Sougrati | first4 = Moulay-Tahar | last5 = Saubanère | first5 = Matthieu | last6 = Courty | first6 = Matthieu | last7 = Recham | first7 = Nadir | last8 = Tarascon | first8 = Jean-Marie | year = 2015 | title = Unraveling the Structure of Iron(III) Oxalate Tetrahydrate and Its Reversible Li Insertion Capability | journal = Chemistry of Materials | volume = 27 | issue = 5| pages = 1631–1639 | doi = 10.1021/cm5043149 }}</ref> Color code: red=O, white = H, blue = Fe, gray = C.|left|360px]]

==Bi- and polymetallic complexes==
Oxalate is often a bridging ligand forming bi- and polynuclear complexes with (κ<sup>2</sup>,κ'<sup>2</sup>-C<sub>2</sub>O<sub>4</sub>)M<sub>2</sub> cores. Illustrative binuclear complexes are [M<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>5</sub>]<sup>2-</sup> M = Fe(II)<ref>{{cite journal |doi=10.1039/b417251e|title=Bis and Tris(oxalato)ferrate(III) Complexes as Precursors of polynuclear compounds|year=2005|last1=Armentano|first1=Donatella|last2=De Munno|first2=Giovanni|last3=Lloret|first3= Francesc|last4=Julve|first4=Miguel|journal=CrystEngComm|volume=7|issue=7|page=57}}</ref> and Cr(III)<ref>{{cite journal |doi=10.1039/a705265k|title=Synthesis, Structure and Magnetism of the Oxalato-Bridged Chromium(III) Complex &#91;NBu<sup>n</sup><sub>4</sub>&#93;<sub>4</sub>&#91;Cr<sub>2</sub>(ox)<sub>5</sub>&#93;·2CHCl<sub>3</sub>|year=1998|last1=Masters|first1=Vanessa M.|last2=Sharrad|first2=Clint A.|last3=Bernhardt|first3=Paul V.|last4=Gahan|first4=Lawrence R.|last5=Moubaraki|first5=Boujemaa|last6=Murray|first6=Keith S.|journal=Journal of the Chemical Society, Dalton Transactions|issue=3|pages=413–416}}</ref>

With only oxalate and water, many metals form polynuclear complexes.<ref>{{cite journal |doi=10.1039/c0cs00111b |title=Multifunctionality in Hybrid Magnetic Materials Based on Bimetallic Oxalate Complexes |date=2011 |last1=Clemente-León |first1=Miguel |last2=Coronado |first2=Eugenio |last3=Martí-Gastaldo |first3=Carlos |last4=Romero |first4=Francisco M. |journal=Chemical Society Reviews |volume=40 |issue=2 |pages=473–497 |pmid=21210054 }}</ref> In such materials, oxalate typically functions as a κ<sup>2</sup>-bidentate ligand, as illustrated by [[ferrioxalate]], {{chem2|[Fe(C2O4)3](3-)}}.  The Δ and Λ enantiomorphs of this trianion have been separated. In ferric oxalate, {{chem2|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 ligand|bridging]].
In ferric oxalate, {{chem2|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 ligand|bridging]].

==Reactions and applications==
[[File:Potassium trioxalatoferrate(III) large crystals.jpg|thumb|left|[[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 [[redox|reduced]] (gains an electron) from the +3 to the +2 [[oxidation state]], while an oxalate ion is oxidised to [[carbon dioxide]]:
: 2 [{{chem|Fe|(|C|2|O|4|)|3}}]<sup>3−</sup> + ''hν'' → 2 [{{chem|Fe|(|C|2|O|4|)|2}}]<sup>2−</sup> + 2 {{chem|C|O|2}} + {{chem|C|2|O|4|2−}}
The redox reaction has been used to access unusual complexes. UV-irradiation of Pt(C<sub>2</sub>O<sub>4</sub>)(PPh<sub>3</sub>)<sub>2</sub> gives derivatives of Pt<sup>0</sup>(PPh<sub>3</sub>)<sub>2</sub>.

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.<ref>{{cite journal |doi=10.1021/acs.iecr.9b02598 |title=Metal Recovery Using Oxalate Chemistry: A Technical Review |date=2019 |last1=Verma |first1=Ankit |last2=Kore |first2=Rajkumar |last3=Corbin |first3=David R. |last4=Shiflett |first4=Mark B. |journal=Industrial & Engineering Chemistry Research |volume=58 |issue=34 |pages=15381–15393 |hdl=1808/31642 |s2cid=201229206 |hdl-access=free }}</ref>

==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.<ref>{{cite journal |doi=10.1080/00958972.2014.937340 |title=Review: Natural oxalates and their analogous synthetic complexes |date=2014 |last1=Baran |first1=Enrique J. |journal=Journal of Coordination Chemistry |volume=67 |issue=23–24 |pages=3734–3768 }}</ref>

==See also==
*[[Transition metal carboxylate complex]]
*[[Oxalatonickelate]]

==References==
<references />

{{Coordination complexes}}
{{oxalates}}

[[Category:Ligands]]
[[Category:Oxalato complexes]]