Tetraiodonickelate

Tetraiodonickelate(II) is a complex ion of nickel with four iodide ions [NiI₄]²⁻ arranged in a tetrahedron.^[1] [NiI₄]²⁻ is red in solution. This colour is due to absorption around 530 nm and below 450 nm. Maximum light transmission is around 620 nm, which is red. A broad weak absorption in the near infrared is at 740 nm.^[1] The magnetic moment is anomalously low.^[2]

A mixture of lithium iodide and nickel iodide in water or methanol can partition [NiI₄]²⁻ ions into a cyclohexane-amine mixture. The solution formed is blood red.^[3]

Already in 1909 Cambi had noticed that a mixture of nickel iodide and sodium iodide dissolved in acetone has a red colour. This red colour was due to the presence of tetraiodonickelate.^[1]

Bis(triphenylmethylarsonium) tetraiodoronickelate [(C₆H₅)₃CH₃As]₂[NiI₄]^[4] is red in colour.^[1] It can be made from triphenylmethylarsonium iodide and nickel iodide in hot ethanol. The red flakes that precipitate must be filtered before the alcohol cools, else the compound decomposes.^[1]

Bis(tetraethylammonium) tetraiodonickelate [(C₂H₅)₄N]₂[NiI₄] has a molecular weight of 826.8135 and a CAS number of 13927-28-1.^[5]

1,2,6-Trimethylpyrazinium tetraiodonickelate has CAS 88227-96-7.^{[citation needed]}