Nickel compounds are chemical compounds containing the element nickel which is a member of the group 10 of the periodic table. Most compounds in the group have an oxidation state of +2. Nickel is classified as a transition metal with nickel(II) having much chemical behaviour in common with iron(II) and cobalt(II). Many salts of nickel(II) are isomorphous with salts of magnesium due to the ionic radii of the cations being almost the same. Nickel forms many coordination complexes. Nickel tetracarbonyl was the first pure metal carbonyl produced, and is unusual in its volatility. Metalloproteins containing nickel are found in biological systems.
Nickel forms simple binary compounds with non metals including halogens, chalcogenides, and pnictides. Nickel ions can act as a cation in salts with many acids, including common oxoacids. Salts of the hexaaqua ion (Ni2+) are especially well known. Many double salts containing nickel with another cation are known. There are organic acid salts. Nickel can be part of a negatively charged ion (anion) making what is called a nickellate. Numerous quaternary compounds (with four elements) of nickel have been studied for superconductivity properties, as nickel is adjacent to copper and iron in the periodic table can form compounds with the same structure as the high-temperature superconductors that are known.
Colour Most of the common salts of nickel are green due to the presence of hexaaquanickel(II) ion, Ni(H2O)62+.
Geometry Nickel atoms can connect to surrounding atoms or ligands in a variety of ways. Six coordinated nickel is the most common and is octahedral, but this can be distorted if ligands are not equivalent. For four coordinate nickel arrangements can be square planar, or tetrahedral. Five coordinated nickel is rarer.
Magnetism Some nickel compounds are ferromagnetic at sufficiently low temperatures. In order to show magnetic properties the nickel atoms have to be close enough together in the solid structure.
Binary compounds A binary compound of nickel contains one other element. Substances that contain only nickel atoms are not actually compounds.
In a noble gas matrix, nickel can form dimers, a molecule with two nickel atoms: Ni2. Ni2 has a bonding energy of 2.07±0.01 eV. For Ni2+ the bond energy is around 3.3 eV. Nickel dimers and other clusters can also be formed in a gas and plasma phase by shooting a powerful laser at a nickel rod in cold helium gas.
Oxides Nickel oxides include Nickel(II) oxide and Nickel(III) oxide.
Hydroxides Nickel hydroxides are used in nickel–cadmium and Nickel–metal hydride batteries. Nickel(II) hydroxide Ni(OH)2, the main hydroxide of nickel is coloured apple green. It is known as the mineral theophrastite. β-NiO(OH) is a black powder with nickel in the +3 oxidation state. It can be made by oxidising nickel nitrate in a cold alkaline solution with bromine. A mixed oxidation state hydroxide Ni3O2(OH)4 is made if oxidation happens in a hot alkaline solution. A Ni4+ hydroxide: nickel peroxide hydrate NiO2, can be made by oxidising with alkaline peroxide. It is black, and unstable and oxidises water.
Halides
Nickel(II) fluoride NiF2 is yellow, crystallising in the rutile structure and can form a trihydrate, NiF2·3H2O. A tetrahydrate also exists.
Nickel(III) fluoride NiF3 and Nickel(IV) fluoride NiF4 also exist.
Nickel chloride NiCl2 is yellow, crystallising in the cadmium chloride structure. It can form a hexahydrate, NiCl2·6H2O, a tetrahydrate NiCl2·4H2O over 29 °C and a dihydrate, NiCl2·2H2O over 64 °C. Ammine complexes like hexaamminenickel chloride also exist.
Nickel bromide NiBr2 is yellow, also crystallising in the cadmium chloride structure. It can form a hexahydrate, NiBr2·6H2O. Crystallisation above 29° forms a trihydrate NiBr2·3H2O, and a dihydrate NiBr2·2H2O.Nicholls p1126-1127 Nonahydrate, NiBr2·9H2O can crystallise from water below 2 °C. Nickelous hexammine bromide Ni(NH3)6Br2is violet or blue. It is soluble in boiling aqueous ammonia, but is insoluble in cold. Diammine, monoammine, and dihydrazine nickel bromides also exist.
With four bromide atoms, nickel(II) forms a series of salts called tetrabromonickelates.
Nickel iodide NiI2 is black, also crystallising in the cadmium chloride structure. It can form a green hexahydrate, NiI2·6H2O. Nickel iodide has a brown diammine NiI2•2NH3 and a bluish-violet hexammine NiI2•6NH3.
Chalcogenides
By reacting nickel with chalcogens, nickel sulfide, nickel selenide, and nickel telluride are formed. There are numerous sulfides: Ni1.5S, Ni17S18, Ni3S2 (heazlewoodite), Ni3S4 (polydymite), Ni9S8 (godlevskite), NiS (millerite) and two other NiS forms, NiS2 (vaesite) in pyrite structure. Black nickel tetrasulfide NiS4 is formed from ammonium polysulfide and nickel in water solution. Mixed and double sulfides of nickel also exist. Nickel with selenium forms several compounds Ni1−xSe 0≤x≤0.15, Ni2Se3, NiSe2 also known as a mineral penroseite.
Nickel forms two different polonides by heating nickel and polonium together: NiPo and NiPo2.
Pnictides
Non-stoichiometric compounds of nickel with phosphorus, arsenic and antimony exist, and some are found in nature. One interstitial nitride has formula Ni3N (hexagonal P6322, Z = 2, a = 4.6224 Å and c = 4.3059 Å). In a solid nitrogen matrix, nickel atoms combine with nitrogen molecules to yield Ni(N2)4.
Nickel phosphide Ni2P has density 7.33 and melts at 1100 °C.
The mineral Nickelskutterudite has formula NiAs2-3, nickeline has formula NiAs and breithauptite has formula NiSb. NiAs melts at 967° and has density 7.77. NiSb melts at 1174°. It has the highest density of a nickel compound at 8.74 g/cm3.
NiAsS gersdorffite, and NiSbS ullmannite, NiAsSe Jolliffeite are pnictide/chalcogenide compounds that occur as minerals.
Other Nickel also forms carbides and borides. Nickel borides include forms Ni2B (a green/black solid), NiB, Ni3B, o-Ni4B3 and m-Ni4B3.
Nickel hydride NiH is only stable under high pressures of hydrogen.
Nickel silicides include Ni3Si, Ni31Si12, Ni2Si, Ni3Si2, NiSi and NiSi2. Nickel silicides are used in microelectronics.
Diatomic molecules Hot nickel vapour reacting with other atoms in the gas phase can produce molecules consisting of two atoms. These can be studied by their emission spectrum. The nickel monohalides are well studied.
Alloys
Compounds of nickel with other metals can be called alloys. The substances with fixed composition include nickel aluminide (NiAl) melting at 1638° with hexagonal structure. NiY, NiY3, Ni3Y, Ni4Y, NiGd3,
BaNi2Ge2 changes structure from orthorhombic to tetragonal around 480 °C. This is a ternary intermetallic compound. Others include BaNiSn3 and the superconductors SrNi2Ge2, SrNi2P2, SrNi2As2, BaNi2P2, BaNi2As2.
Simple salts
Oxo acid salts
Important nickel oxo acid salts include nickel(II) sulfate can crystallise with six water molecules yielding Retgersite or with seven making Morenosite which is isomorphic to Epsom salts. These contain the hexaquanickel(II) ion. There is also an anhydrous form, a dihydrate and a tetrahydrate, the last two crystallised from sulfuric acid. The hexahydrate has two forms, a blue tetragonal form, and a green monoclinic form, with a transition temperature around 53 °C.Mellor pp462-465 The heptahydrate crystallises from water below 31.5 above this blue hexahydrate forms, and above 53.3 the green form.Mellor p 466-467 Heating nickel sulfate dehydrates it, and then 700° it loses sulfur trioxide, sulfur dioxide and oxygen. Other important nickel compounds in this class are nickel carbonate, nickel nitrate, and nickel phosphate
Fluoro acid salts Nickel tetrafluoroborate, Ni(BF4)2 is very soluble in water, alcohol and acetonitrile. It is prepared by dissolving nickel carbonate in tetrafluoroboric acid. Nickel tetrafluoroberyllate NiBeF4•xH2O, can be hydrated with six or seven water molecules. Both nickel hexafluorostannate NiSnF6.6H2O and nickel fluorosilicate NiSiF6.6H2O crystallise in the trigonal system. Nickel hexafluorogermanate NiGeF6 has a rosy-tan colour and a hexagonal crystal with a = 5.241 Å unit cell volume is 92.9 Å3. It is formed in the reaction with GeF4 and K2NiF6. Nickel fuorotitanate NiTiF6.6H2O crystallises in hexagonal green crystals. It can be made by dissolving nickel carbonate, and titanium dioxide in hydrofluoric acid. The crystal dimensions are a = 9.54, c = 9.91 density = 2.09 (measure 2.03).
Ni(AsF6)2, Ni(SbF6)2, Ni(BiF6)2 are made by reacting the hexafluoro acid with NiF2 in hydrofluoric acid. They all have hexagonal crystal structure, resembling the similar salts of the other first row transition metals. For Ni(AsF6)2 a = 4.98, c = 26.59, and V = 571, formula weight Z=3. Ni(SbF6)2 is yellow with a = 5.16Å, c = 27.90Å Z = 3. The structure resembles LiSbF6, but with every second metal along the c axis missing.
Others include the green fluorohafnate NiHfF6, and Ni2HfF8, NiZrF6
Chloroacid salts Nickel tetrachloroiodate Ni(ICl4)2 can be made by reacting iodine with nickel chloride and chlorine gas. It consists of green needles.
Nitrogen anion salts Nickel cyanide tetrahydrate Ni(CN)2 is insoluble in water, but dissolves in aqueous ammonia. It forms double salts with interesting structures.
Nickel azide Ni(N3)2 is a sensitive explosive. It can be made by treating nickel carbonate with hydrazoic acid. Acetone causes the precipitation of the hydrous solid salt, which is green. At 490K it slowly decomposes to nitrogen and nickel metal powder, losing a half of the nitrogen in four hours. Nickel azide is complexed by one azo group when dissolved in water, but in other solvents, the nickel atom can have up to four azo groups attached. Nickel azide forms a dihydrate: Ni(N3)2 and a basic salt called nickel hydroxy azide Ni(OH)N3.
Nickel amide, Ni(NH2)2 is a deep red compound that contains Ni6 clusters surrounded by 12 NH2 groups. Nickel amide also forms a series of double salts. Other homoleptic nickel amides derived by substituting the hydrogen atoms are Ni[N(C6H5)2]2 (diphenyl) and boryl amides Ni[NBMes2Mes]2 and Ni[NBMes2C6H5]2.
Organic acid salts
Nickel forms many known salts with organic acids. In many of these the ionised organic acid acts as a ligand.
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 (NH4), or thallium. As a mineral the ammonium nickel salt, (NH4)2Ni(SO4)2, can be called nickelboussingaultite. With sodium, the double sulfate is nickelblödite Na2Ni(SO4)2 from the blödite family. Nickel can be substituted by other divalent metals of similar sized to make mixtures that crystallise in the same form.
Nickel forms double salts with Tutton's salt structure with tetrafluoroberyllate with the range of cations of ammonia, potassium, rubidium, cesium, and thallium.
Anhydrous salts of the formula M2Ni2(SO4)3, which can be termed metal nickel trisulfates, belong to the family of langbeinites. The known salts include (NH4)2Ni2(SO4)3, K2Ni2(SO4)3 and Rb2Ni2(SO4)3, and those of Tl and Cs are predicted to exist.
Some minerals are double salts, for example Nickelzippeite Ni2(UO2)6(SO4)3(OH)10 · 16H2O which is isomorphic to cobaltzippeite, magnesiozippeite and zinczippeite, part of the zippeite group.
Double hydrides of nickel exist, such as Mg2NiH4.
Ternary chalcogenides Nickel forms a series of double nickel oxides with other elements, which may be termed "nickelates". There are also many well defined double compounds with sulfur, selenium and tellurium.
Ternary pnictides Ternary pnictides that contain nickel are metallic and include MgNi2Bi4, SrNi2P2, SrNi2As2, BaNi2P2, and BaNi2As2.
Ternary halides Nickel can form anions and salts with halogens including the hexafluoronickelates, and tetrafluoronickelates, tetrachloronickelates, tetrabromonickelates and tetraiodonickelates. The subiodide Bi12Ni4I3 is also known.
Polyoxometallates