Cobalt is a chemical element; it has symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, somewhat brittle, gray metal.
Cobalt-based blue pigments (cobalt blue) have been used since antiquity for jewelry and paints, and to impart a distinctive blue tint to glass. The color was long thought to be due to the metal bismuth. Miners had long used the name kobold ore (German for goblin ore) for some of the blue pigment-producing minerals. They were so named because they were poor in known metals and gave off poisonous arsenic-containing fumes when smelted. In 1735, such ores were found to be reducible to a new metal (the first discovered since ancient times), which was ultimately named for the kobold.
Today, cobalt is usually produced as a by-product of copper and nickel mining, but sometimes also from one of a number of metallic-lustered ores such as cobaltite (CoAsS). The Copperbelt in the Democratic Republic of the Congo (DRC) and Zambia yields most of the global cobalt production. World production in 2016 was according to Natural Resources Canada, and the DRC alone accounted for more than 50%. In 2024, production exceeded 300,000 tons, of which DRC accounted for more than 80%.
Cobalt is primarily used in lithium-ion batteries, and in the manufacture of magnetic, wear-resistant and high-strength alloys. The compounds cobalt silicate and cobalt(II) aluminate (, cobalt blue) give a distinctive deep blue color to glass, ceramics, inks, paints and varnishes. Cobalt occurs naturally as only one stable isotope, cobalt-59. Cobalt-60 is a commercially important radioisotope, used as a radioactive tracer and for the production of high-energy gamma rays. Cobalt is also used in the petroleum industry as a catalyst when refining crude oil. This is to purge it of sulfur, which is very polluting when burned and causes acid rain.
Cobalt is the active center of a group of coenzymes called cobalamins, also known as Vitamin B12, which is an essential vitamin for all animals. Cobalt in inorganic form is also a micronutrient for bacteria, algae, and fungi.
The name cobalt derives from a type of ore considered a nuisance by 16th-century German silver miners, which in turn may have been named from a spirit or goblin held superstitiously responsible for it; this spirit is considered equitable to the kobold (a household spirit) by some, or categorized as a gnome (mine spirit) by others.
Characteristics
Cobalt is a ferromagnetic metal with a specific gravity of 8.9. The Curie temperature is and the magnetic moment is 1.6–1.7 Bohr magnetons per atom. Cobalt has a relative permeability two-thirds that of iron. Metallic cobalt occurs as two crystallographic structures: hcp and fcc. The ideal transition temperature between the hcp and fcc structures is , but in practice the energy difference between them is so small that random intergrowth of the two is common.
Cobalt is a weakly reducing metal that is protected from oxidation by a passivating oxide film. It is attacked by halogens and sulfur. Heating in oxygen produces which loses oxygen at to give the monoxide CoO. The metal reacts with fluorine () at 520 K to give ; with chlorine (), bromine () and iodine (), producing equivalent binary halides. It does not react with hydrogen gas () or nitrogen gas () even when heated, but it does react with boron, carbon, phosphorus, arsenic and sulfur. At ordinary temperatures, it reacts slowly with mineral acids, and very slowly with moist, but not dry, air.
Compounds
Common oxidation states of cobalt include +2 and +3, although compounds with oxidation states ranging from −3 to +5 are also known. A common oxidation state for simple compounds is +2 (cobalt(II)). These salts form the pink-colored metal aquo complex in water. Addition of chloride gives the intensely blue . In a borax bead flame test, cobalt shows deep blue in both oxidizing and reducing flames.
Oxygen and chalcogen compounds Several binary oxides of cobalt are known. Green cobalt(II) oxide (CoO) has rocksalt structure. It is readily oxidized with water and oxygen to brown cobalt(III) hydroxide (). At temperatures of 600–700 °C, CoO oxidizes to the blue cobalt(II,III) oxide (), which has a spinel structure. Black cobalt(III) oxide () is also known. Cobalt oxides are antiferromagnetic at low temperature: CoO (Néel temperature 291 K) and (Néel temperature: 40 K), which is analogous to magnetite (), with a mixture of +2 and +3 oxidation states. Potassium percobaltate () contains a rare example of a cobalt(V) complex.
The principal chalcogenides of cobalt are the black cobalt(II) sulfides, (pyrite structure), (spinel structure), and CoS (nickel arsenide structure).
Halides
The salt caesium hexafluorocobaltate(IV) () is a rare example of cobalt(IV). The reduction potential for the reaction is +1.92 V, beyond that for chlorine to chloride, +1.36 V. Consequently, cobalt(III) chloride would spontaneously reduce to cobalt(II) chloride and chlorine. Because the reduction potential for fluorine to fluoride is so high, +2.87 V, cobalt(III) fluoride is one of the few simple stable cobalt(III) compounds. Cobalt(III) fluoride, which is used in some fluorination reactions, reacts vigorously with water.
Four dihalides of cobalt(II) are known: cobalt(II) fluoride (, pink), cobalt(II) chloride (, blue), cobalt(II) bromide (, green), cobalt(II) iodide (, blue-black). These halides also exist in hydrated forms. Whereas the anhydrous dichloride is blue, the hydrate is red. The pink-colored cation hexaaquocobalt(II) is found in several routine cobalt salts such as the nitrate and sulfate.
Coordination complexes A large inventory of complexes exists for cobalt(III) bound to ammonia and amines (called ammine complexes). Examples include , (chloropentamminecobalt(III)), and cis- and trans-. The corresponding ethylenediamine complexes are also well known. Analogues are known where the halides are replaced by nitrite, hydroxide, carbonate, etc. Alfred Werner worked extensively on these complexes in his Nobel-prize winning work. The robustness of these complexes is demonstrated by the optical resolution of tris(ethylenediamine)cobalt(III) ().
The unusually high oxidation state +IV of cobalt can be stabilized by ligands based on in dark blue colored crystals. Softer ligands like triphenylphosphine form complexes with Co(II) and Co(I), examples being bis- and tris(triphenylphosphine)cobalt(I) chloride, and . These Co(I) and Co(II) complexes represent a link to the organometallic complexes described below.
Organometallic compounds
Cobaltocene is a structural analog to ferrocene, with cobalt in place of iron. Cobaltocene is much more sensitive to oxidation than ferrocene. Cobalt carbonyl () is a catalyst in carbonylation and hydrosilylation reactions. Vitamin B12 (see below) is an organometallic compound found in nature and is the only vitamin that contains a metal atom. An example of an alkylcobalt complex in the otherwise uncommon +4 oxidation state of cobalt is the homoleptic complex tetrakis(1-norbornyl)cobalt(IV) (), a transition metal-alkyl complex that is notable for its resistance to β-hydrogen elimination, in accord with Bredt's rule. The cobalt(III) and cobalt(V) complexes and are also known.
Isotopes
Co is the only stable cobalt isotope and the only isotope that exists naturally on Earth. Twenty-two radioisotopes have been characterized: the most stable, Co, has a half-life of 5.2714 years; Co has a half-life of 271.81 days; Co has a half-life of 77.24 days; and Co has a half-life of 70.84 days. All the other radioactive isotopes of cobalt have half-lives shorter than 18 hours, and in most cases shorter than 1 second. This element also has 4 meta states, all of which have half-lives shorter than 15 minutes.
The isotopes of cobalt range from Co to Co. The primary decay mode for isotopes with atomic masses less than that of the only stable isotope, Co, is electron capture and the primary mode of decay in isotopes with atomic mass greater than that is beta decay. The primary decay products below Co are element 26 (iron) isotopes; above that the decay products are element 28 (nickel) isotopes.
The Co nucleus is detectable using nuclear magnetic resonance and has a magnetic quadrupole moment. Among all NMR active nuclei, Co has the largest chemical shift range and the chemical shift can be correlated with the spectrochemical series. Resonances are observed over a range of 20000 ppm, the width of the signals being up to 20 kHz. A widely used standard is potassium hexacyanocobaltate (0.1M in ), which, due to its high symmetry, has a rather small line width. Systems of low symmetry can yield broadened signals to an extent that renders the signals unobservable in fluid phase NMR, but still observable in solid state NMR.
Etymology
Many different stories about the origin of the word "cobalt" have been proposed. In one version the element cobalt was named after "", the name which 16th century German silver miners had given to a nuisance type of ore which occurred that was corrosive and issued poisonous gas. Although such ores had been used for blue pigmentation since antiquity, the Germans at that time did not have the technology to smelt the ore into metal (cf. below).
The authority on such kobelt ore (Latinized as cobaltum or cadmia) at the time was Georgius Agricola. He was also the oft-quoted authority on the mine spirits called "" (Latinized as cobalus or pl. cobali) in a separate work.This passage from the separate work, de animantibus is translated in footnote by the , : "the Germans as well as the Greeks call cobalos".
Agricola did not make a connection between the similarly named ore and spirit. However, a causal connection (ore blamed on "kobel") was made by a contemporary, and a word origin connection (word "formed" from cobalus) made by a late 18th century writer. Later, Grimms' dictionary (1868) noted the kobalt/kobelt ore was blamed on the mountain spirit () which was also held responsible for "stealing the silver and putting out an ore that caused poor mining atmosphere (Wetter) and other health hazards".
Grimms' dictionary entries equated the word "kobel" with "kobold", and listed it as a mere variant diminutive, but the latter is defined in it as a household spirit. Some more recent commentators prefer to characterize the ore's namesake kobelt (recté kobel) as a gnome.
The early 20th century Oxford English Dictionary (1st edition, 1908) upheld Grimm's etymology. However, by around the same time in Germany, the alternate etymology not endorsed by Grimm (kob/kof "house, chamber" + walt "power, ruler") was being proposed as more convincing.
Somewhat later, Paul Kretschmer (1928) explained that while this "house ruler" etymology was the proper one that backed the original meaning of kobold as household spirit, a corruption later occurred introducing the idea of "mine demon" to it. The present edition of the Etymologisches Wörterbuch (25th ed., 2012) under "kobold" lists the latter, not Grimm's etymology, but still maintains, under its entry for "kobalt", that the cobalt ore may have gotten its name from "a type of mine spirit/demon" (daemon metallicus) while stating that this is "apparently" the kobold.
Joseph William Mellor (1935) also stated that cobalt may derive from kobalos (), an ancient Greek sprite, though other theories had been suggested.
Alternate theories Several alternative etymologies that have been suggested, which may not involve a spirit (kobel or kobold) at all. Karl Müller-Fraureuth conjectured that kobelt derived from , a bucket used in mining, frequently mentioned by Agricola, namely the kobel/köbel (Latinized as modulus).Agricola (1546) :