The oxalatonickelates are a class of compounds that contain nickel complexed by oxalate groups. They form a series of double salts, and include clusters with multiple nickel atoms. Since oxalate functions as a bidentate ligand it can satisfy two coordinate positions around the nickel atom, or it can bridge two nickel atoms together.
The shape around the nickel atom is octahedral for diaquabis(oxalato)nickelate. The colour of this is green due to the Ni(–O–)6 chromophore. The absorption in infrared is at 8,300 9,060 13,400, 15,260, and 26,160 cm−1.[{{cite journal|last1=Morrison|first1=J.T.|last2=Baker|first2=W.A.|title=Nickel(II) complexes with oxygen and sulfur donors|journal=Inorganica Chimica Acta|date=January 1969|volume=3|pages=463–467|doi=10.1016/S0020-1693(00)92534-9}}] The ligand field parameters are 10 Dq=8800 cm−1 and B=1000 cm−1.
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| [(C2O4)2Ni]2−
|bis(oxalato)nickelate (2−)
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|[{{cite book|last1=Macintyre|first1=Jane E.|title=Dictionary of Inorganic Compounds, Supplement 4|date=1997|publisher=CRC Press|isbn=9780412750205|page=453|url=https://books.google.com/books?id=VvDoE5oN09wC&pg=PA453|accessdate=21 April 2016|language=en}}] |
| Li2[(C2O4)2Ni]•6H2O
|dilithium bis(oxalato)nickelate (2−)
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|dehydrate 80° decompose 345°
|112678-94-1
|[{{cite journal|last1=Saha|first1=H.L.|last2=Mitra|first2=S.|title=Thermal decomposition reactions of metal carboxylato complexes in the solid state. III. Thermographic and differential thermal studies of metal oxalato, malonato and succinato complexes|journal=Thermochimica Acta|date=June 1987|volume=116|pages=53–64|doi=10.1016/0040-6031(87)88164-9|bibcode=1987TcAc..116...53S }}] |
| Na2[(C2O4)2Ni]•3H2O
|disodium bis(oxalato)nickelate (2−)
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|dehydrates at 85°, decompose over 320°
|107996-66-5
|[{{cite book|last1=Macintyre|first1=Jane E.|title=Dictionary of Inorganic Compounds, Supplement 4|date=1997|publisher=CRC Press|isbn=9780412750205|page=251|url=https://books.google.com/books?id=VvDoE5oN09wC&pg=PA453|accessdate=21 April 2016|language=en}}][{{cite journal|last1=Saha|first1=H.L.|last2=Mitra|first2=S.|title=Thermal decomposition reactions of metal carboxylato complexes in the solid state. I. Thermographic and differential thermal studies of metal oxalato, malonato and succinato complexes|journal=Thermochimica Acta|date=January 1987|volume=109|issue=2|pages=331–342|doi=10.1016/0040-6031(87)80029-1|bibcode=1987TcAc..109..331S }}] |
| [NH4]2[(C2O4)2Ni]•3H2O
|diammonium bis(oxalato)nickelate (2−)
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|dehydrate 180-262° deaminate 262-338 °C
|108559-31-5
|[{{cite journal|last1=Saha|first1=H.L.|last2=Mitra|first2=S.|title=Thermal decomposition reactions of metal carboxylato complexes in the solid state. II. Thermographic and differential thermal studies of metal oxalato, malonato and succinato complexes|journal=Thermochimica Acta|date=March 1987|volume=112|issue=2|pages=275–287|doi=10.1016/0040-6031(87)88284-9|bibcode=1987TcAc..112..275S }}] |
| K2[(C2O4)2Ni•2H2O]•4H2O
|dipotassium trans-diaquabis(oxalato-O,O')nickelate(II)-water (1/4)
|potassium bis oxalate nickel(II) tetrahydrate
|monoclinic a=8.647 b=6.627 c=12.118 β=101.58° V=680.3 Z=2 density=2.05 / 2.04
|green
|14244-63-4
|[{{cite journal|last1=Román|first1=P.|last2=Guzmán-Miralles|first2=C.|last3=Luque|first3=A.|title=Structure of dipotassium trans-diaquabis(oxalato-O,O')nickelate(II)–water (1/4)|journal=Acta Crystallographica Section C|date=15 July 1993|volume=49|issue=7|pages=1336–1339|doi=10.1107/S0108270193000940|bibcode=1993AcCrC..49.1336R |url=http://journals.iucr.org/c/issues/1993/07/00/ab1046/ab1046.pdf}}][{{cite journal|last1=Narsimhulu|first1=M.|last2=Raju|first2=B.|last3=Saritha|first3=A.|last4=Narayana Rao|first4=D.|last5=Hussain|first5=K.A.|title=A new room-temperature ultraviolet emission material: |journal=Physica B: Condensed Matter|date=September 2015|volume=472|pages=45–48|doi=10.1016/j.physb.2015.05.019|bibcode=2015PhyB..472...45N}}][{{cite book|last1=Macintyre|first1=Jane E.|title=Dictionary of Inorganic Compounds, Supplement 1|publisher=CRC Press|isbn=9780412490903|page=45|url=https://books.google.com/books?id=SJepeRehoTIC&pg=PA45|accessdate=25 April 2016|language=en|date=1993-07-15}}][{{cite book|last1=Macintyre|first1=Jane E.|title=Dictionary of Inorganic Compounds, Supplement 4|date=1997|publisher=CRC Press|isbn=9780412750205|page=222|url=https://books.google.com/books?id=VvDoE5oN09wC&pg=PA453|accessdate=21 April 2016|language=en}}] |
| Co[(C2O4)2Ni]•5H2O
|cobalt(II)bis(oxalato)nickelate(II)pentahydrate
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|light pink
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|[{{cite journal|last1=Deb|first1=Nidhuban|last2=Baruah|first2=S. D.|last3=Dass|first3=N. N.|title=Synthesis, Characterization and Thermal Decomposition of ( and )|journal=Journal of Thermal Analysis and Calorimetry|date=2000|volume=59|issue=3|pages=791–797|doi=10.1023/A:1010153720786|s2cid=94453246}}] |
| Mn[(C2O4)2Ni]•4H2O
|manganese(II)bis(oxalato)nickelate(II)tetrahydrate
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|light blue
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|[{{cite journal|last1=Deb|first1=Nidhuban|title=Thermal decomposition of manganese(II)bis(oxalato)nickelate(II)tetrahydrate|journal=Journal of Thermal Analysis and Calorimetry|date=July 2005|volume=81|issue=1|pages=61–65|doi=10.1007/s10973-005-0746-y|s2cid=94799986}}] |
| Cd[(C2O4)2Ni]•4H2O
|cadmium(II)bis(oxalato)nickelate(II)tetrahydrate
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|blue
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Because the Ni2+ ion resembles many other divalent metal ions, it can be substituted by them, or substitute for them in other oxalate compounds to form mixed oxalates where the proportions can vary continuously. For example, magnesium nickel oxalate dihydrate.[{{cite journal|last1=Packter|first1=A.|last2=Omomo|first2=A.|title=The coprecipitation of Magnesium Nickel Oxalate Dihydrate Powders (solid solutions) from aqueous solution: Precipitate compositions and Coprecipitate Mechanisms|journal=Crystal Research and Technology|date=1984|volume=19|issue=4|pages=467–476|doi=10.1002/crat.2170190406|bibcode=1984CryRT..19..467P }}]