Nickel compounds
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 having much chemical behaviour in common with iron and cobalt. Many salts of nickel 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 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 making what is called a nickellate. Numerous quaternary compounds 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 ion, Ni62+.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 oxide and Nickel oxide.Hydroxides
Nickel hydroxides are used in nickel–cadmium and Nickel–metal hydride batteries.Nickel hydroxide Ni2, the main hydroxide of nickel is coloured apple green. It is known as the mineral theophrastite.
β-NiO 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 Ni3O24 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
NiF2 is yellow, crystallising in the rutile structure and can form a trihydrate, NiF2·3H2O. A tetrahydrate also exists.Nickel fluoride NiF3 and Nickel 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. Nonahydrate, NiBr2·9H2O can crystallise from water below 2 °C. Nickelous hexammine bromide Ni6Br2is 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 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, Ni3S4, Ni9S8, NiS and two other NiS forms, NiS2 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
compounds of nickel with phosphorus, arsenic and antimony exist, and some are found in nature. One interstitial nitride has formula Ni3N.In a solid nitrogen matrix, nickel atoms combine with nitrogen molecules to yield Ni4.
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, 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 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 sulfate can crystallise with six water molecules yielding Retgersite or with seven making Morenosite which is isomorphic to Epsom salts. These contain the hexaquanickel 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. The heptahydrate crystallises from water below 31.5 above this blue hexahydrate forms, and above 53.3 the green form. 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, Ni2 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.
Ni2, Ni2, Ni2 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 Ni2 a = 4.98, c = 26.59, and V = 571, formula weight Z=3. Ni2 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