Nickel can form various double salts.
Nickel is one of the metals that can form Tutton's salts. The singly charged ion can be any of the full range of potassium, rubidium, cesium, ammonium (NH<sub>4</sub>), or thallium.
As a mineral the ammonium nickel salt, (NH<sub>4</sub>)<sub>2</sub>Ni(SO<sub>4</sub>)<sub>2</sub>, can be called nickelboussingaultite. With sodium, the double sulfate is nickelblödite Na<sub>2</sub>Ni(SO<sub>4</sub>)<sub>2</sub> from the blödite family. Nickel can be substituted by other divalent metals of similar sized to make mixtures that crystallise in the same form.
Anhydrous salts of the formula M<sub>2</sub>Ni<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>, which can be termed metal nickel trisulfates, belong to the family of langbeinites. The known salts include (NH<sub>4</sub>)<sub>2</sub>Ni<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>, K<sub>2</sub>Ni<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> and Rb<sub>2</sub>Ni<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>, and those of Tl and Cs are predicted to exist.
Double fluorides include the fluoroanion salts, and those fluoronickelates such as NiF<sub>4</sub> and NiF<sub>6</sub>:
Nickel trichloride double salts exist which are polymers. Nickel is in octahedral coordination, with double halogen bridges. Examples of this include RbNiCl<sub>3</sub>, pinkish tan coloured H<sub>2</sub>NN(CH<sub>3</sub>)<sub>3</sub>NiCl<sub>3</sub>.
Other double trichlorides include:
The tetrachloronickelates contain a tetrahedral NiCl<sub>4</sub><sup>2âÂÂ</sup> and are dark blue. Some salts of organic bases are ionic liquids at standard conditions. Tetramethylammonium nickel trichloride is pink and very insoluble.
Other tetrachlorides include:
Double hexachlorides include:
Thallic nickel octochloride 2TlCl<sub>3</sub>÷NiCl<sub>2</sub>÷8H<sub>2</sub>O is bright green.
Copper nickel dioxychloride 2CuO÷NiCl<sub>2</sub>÷6H<sub>2</sub>O and copper nickel trioxychloride 3CuO÷NiCl<sub>2</sub>÷4H<sub>2</sub>O exist.
Double bromides include the tetrabromonickelates. Other salts include:
The tetraiodonickelates are blood-red coloured salts of the NiI<sub>4</sub> ion with large cations. The diperiodatonickelates of nickel(IV) are strong oxidisers, and akali monoperiodatonickelates also are known.
Double iodides include:
Nickel forms double nitrates with the lighter rare-earth elements. The solid crystals have the formula Ni<sub>3</sub>Me<sub>2</sub>(NO<sub>3</sub>)<sub>12</sub>â¢24H<sub>2</sub>O. The metals include La, Ce, Pr, Nd, Sm, Gd and the non rare earth Bi. Nickel can also be replaced by similar divalent ions Mg, Mn, Co, and Zn. For the nickel salts melting temperatures range from 110.5ð for La, 108.5ð for Ce, 108ð for Pr, 105.6ð for Nd, 92.2ð for Sm and down to 72.5ð for Gd, the Bi salt melting at 69ð. Crystal structure is hexagonal with Z=3. Ni<sub>3</sub>La<sub>2</sub>(NO<sub>3</sub>)<sub>12</sub>â¢24H<sub>2</sub>O becomes ferromagnetic below 0.393 K. These double nickel nitrates have been used to separate the rare earth elements by fractional crystallization.
Nickel thorium nitrate has formula NiTh(NO<sub>3</sub>)<sub>6</sub>. Nickel atoms can be substituted by other ions with radius 0.69 to 0.83 à. The nitrates are coordinated on the thorium atom and the water to the nickel. Enthalpy of solution of the octahydrate is 7 kJ/mol. Enthalpy of formation is -4360 kJ/mol. At 109ð the octahydrate becomes NiTh(NO<sub>3</sub>)<sub>6</sub>â¢6H<sub>2</sub>O, and at 190ð NiTh(NO<sub>3</sub>)<sub>6</sub>â¢3H<sub>2</sub>O and anhydrous at 215ð. The hexahydrate has Pa cubic structure.
Various double amides containing nickel clusters have been made using liquid ammonia as a solvent. These are called amidonickel compounds.
These include:
Double hydrides of nickel exist, such as Mg<sub>2</sub>NiH<sub>4</sub>.
Nickel dihydrogen phosphide (Ni(PH<sub>2</sub>)<sub>2</sub>) can form orange, green or black double salts KNi(PH<sub>2</sub>)<sub>3</sub>) that crystallise from liquid ammonia. They are unstable above -78 ðC, giving off ammonia, phosphine and hydrogen.
Nickel forms double salts with Tutton's salt structure with tetrafluoroberyllate and a range of cations including ammonia, potassium, rubidium, cesium, and thallium.
Some minerals are double salts, for example nickelzippeite Ni<sub>2</sub>(UO<sub>2</sub>)<sub>6</sub>(SO<sub>4</sub>)<sub>3</sub>(OH)<sub>10</sub> ÷ 16H<sub>2</sub>O which is isomorphic to cobaltzippeite, magnesiozippeite and zinczippeite, part of the zippeite group.