Oxalatonickelate

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.^[1] The ligand field parameters are 10 Dq=8800 cm⁻¹ and B=1000 cm⁻¹.^[1]

formula	name	other names	structure	colour or decomposition	CAS Number	references
[(C2O4)2Ni]2−	bis(oxalato)nickelate (2−)					[2]
Li2[(C2O4)2Ni]•6H2O	dilithium bis(oxalato)nickelate (2−)			dehydrate 80° decompose 345°	112678-94-1	[3]
Na2[(C2O4)2Ni]•3H2O	disodium bis(oxalato)nickelate (2−)			dehydrates at 85°, decompose over 320°	107996-66-5	[4][5]
[NH4]2[(C2O4)2Ni]•3H2O	diammonium bis(oxalato)nickelate (2−)			dehydrate 180-262° deaminate 262-338 °C	108559-31-5	[4][6]
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	[7][8][9][10]
Co[(C2O4)2Ni]•5H2O	cobalt(II)bis(oxalato)nickelate(II)pentahydrate			light pink		[11]
Mn[(C2O4)2Ni]•4H2O	manganese(II)bis(oxalato)nickelate(II)tetrahydrate			light blue		[12]
Cd[(C2O4)2Ni]•4H2O	cadmium(II)bis(oxalato)nickelate(II)tetrahydrate			blue		[11]

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.^[13]