Tetraiodonickelate is a complex ion of nickel with four iodide ions [NiI<sub>4</sub>]2− arranged in a tetrahedron. [NiI<sub>4</sub>]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. The magnetic moment is anomalously low.[1]
A mixture of lithium iodide and nickel iodide in water or methanol can partition NiI42− ions into a cyclohexane-amine mixture. The solution formed is blood red.[2]
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.[3]
Bis-triphenylmethylarsonium tetraiodoronickelate [(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>CH<sub>3</sub>As]2NiI4 is red in colour.[3] 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.[3]
Bis(tetraethylammonium) tetraiodonickelate has a molecular weight of 826.8135 and a CAS number of 13927-28-1.[4]
1,2,6-trimethyl-pyrazinium-tetraiodonickelate has CAS 88227-96-7.