The tetrabromonickelate anion contains a doubly-charged nickel atom (Ni2+) surrounded by four bromide ions in a tetrahedral arrangement. The formula is [NiBr<sub>4</sub>]2−.
The anion combines with cations to form a series of salts called tetrabromonickelates.Strongly-coordinating solvents will displace one or more of the bromido ligands from the complex.Solvents that can dissolve tetrabromonickelate include acetone, acetonitrile, methyl ethyl ketone, and nitromethane.[1]
In the visible absorption spectrum there is a strong absorption band termed ν3 near 710 nm which is caused by an electronic transition from 3T1(F) → 3T1(P). Another strong absorption in the near infrared called ν2 near 770 nm is due to the 3T1(F) → 3A2(F) transition.[1]
Dilithium tetrabromonickelate forms a dark blue solution in tetrahydrofuran.[2]
A mixture of lithium bromide and nickel bromide in water or methanol can transfer [NiBr<sub>4</sub>]2− ions into a cyclohexane-amine mixture. The solution formed is green.[3]
Tetraethylammonium tetrabromonickelate is blue.[4] [5]
tetra-n-butylammonium tetrabromonickelate is purple blue in color, melting around 83°C. It is formed from nickel bromide and tetra-n-butylammonium bromide in ethyl alcohol.[6]
Tetraphenylphosphonium tetrabromonickelate(II) can be made from nickel bromide, triphenylphosphine, and bromobenzene by heating them together in a sealed tube at 250 °C. This substance is dark blue. If it is heated over 260° the color changes to green, and it melts at 273 °C.[7] [8]
A dark blue oil, bis-(o-tolyltriethylphosphonium) tetrabromonickelate (o-CH3C6H6PEt3)2NiBr4 can be made from the reaction of o-tolyl bis-triethylphosphine, nickel bromide and o-tolyl bromide. (o-CH3OC6H6PEt3)2NiBr4 and (C6H5PEt3)2NiBr4 are made in a similar way.[9]
bis-(benzo[''e'']-1,3-dithiepan-2-diethylimmonium) tetrabromonickelate, abbreviated as (xdtc)2NiBr4, has dark blue crystals that melt at 166 °C. It is made from α,α'-dibromo-o-xylene and bis-(N,N-diethyldithiocarbamato)nickel(II) dissolved in 1,2-dichloroethane.[10]
bis-(tetra-n-butylphosphonium) tetrabromonickelate can form as a glass when nickel bromide is dissolved in molten tetra-n-butylphosphonium iodide.[11]
Blue 1,1,1-trimethylhydrazinium tetrabromonickelate [H<sub>2</sub>NN(CH<sub>3</sub>)<sub>3</sub>]2NiBr4 is a salt stable between 70° and 260 °C. It can be formed by heating an octahedral neutral nickel complex where the trimethylhydrazinium and bromide are all ligands on a nickel atom. The salt decomposes at room temperature to polymeric trimethylhydrazinium nickel tribromide. The salt decomposes when dissolved in polar solvents, due to solvolysis.[12]
One bromine atom can be substituted by other ligands, such as triphenylphosphine, to make a dark green triphenylphosphinetribromonickelate ion.[8] Analogous tetrahedral nickelates include tetrafluoronickelates, tetrachloronickelates, tetraiodonickelates, and tetracyanonickelates.