Cobalt is a hard, lustrous, silvery-gray transition metal. It is ferromagnetic, forms +2 and +3 oxidation states, and is essential in high-strength alloys, batteries, and catalysts.
Ground-state configuration of cobalt metal is \([Ar]3d^7\,4s^2\). The most common oxidation states in compounds are +2 (Co2+, typically pink/blue) and +3 (Co3+, often stabilized in ammine/chelate complexes). Higher states like +4 are rare and strongly oxidizing.
Colors and magnetism arise from d–d transitions and electron pairing under octahedral/tetrahedral crystal fields. For octahedral complexes, the crystal-field splitting \(\Delta_o\) can lead to high-spin or low-spin configurations depending on ligand strength (spectrochemical series). Co3+ with strong-field ligands (e.g., NH3, CN−) is often low-spin and diamagnetic, whereas Co2+ is frequently high-spin and paramagnetic.
Hydrated Co2+ is usually pink (octahedral \([\mathrm{Co(H_2O)_6}]^{2+}\)). In the presence of excess halide or upon dehydration/heating, tetrahedral \([\mathrm{CoCl_4}]^{2-}\) can form, which is blue. The pink ⇌ blue color change is a classic equilibrium used in teaching coordination chemistry.
A schematic half-reaction is:
\(\mathrm{Co^{3+} + e^- \rightleftharpoons Co^{2+}}\)
For example, hexamminecobalt(III) can be reduced to Co(II) by iodide:
\(\mathrm{[Co(NH_3)_6]^{3+} + I^- \rightarrow [Co(NH_3)_6]^{2+} + \tfrac{1}{2}I_2}\)
(overall balanced with appropriate stoichiometry). Co(III) is generally stabilized by strong-field ligands.
Cobalt blue is the pigment cobalt aluminate spinel, \(\mathrm{CoAl_2O_4}\), produced by calcining cobalt(II) oxide with alumina at high temperature. It yields a stable, intense blue used in ceramics, glass, and artist paints due to its lightfastness and chemical durability.
Cobalt is a key component in layered Li-ion cathodes such as NMC (LiNi–Mn–Co–O) and NCA (LiNi–Co–Al–O). Co helps stabilize the layered structure, improves cycle life, and enhances rate capability. Research aims to reduce Co content for cost/ethics while retaining performance.
Co-based and Co-containing superalloys maintain strength, creep resistance, and corrosion/oxidation resistance at elevated temperatures, useful in turbines and aerospace. Cobalt also appears in cemented carbides as a binder phase (e.g., WC–Co) providing toughness and wear resistance for cutting tools.
Cobalamin contains a corrin macrocycle coordinated to Co, forming organometallic Co–C bonds in coenzyme forms (e.g., methylcobalamin, adenosylcobalamin). The cobalt center cycles between Co(I)/Co(II)/Co(III) states to mediate methyl transfer and rearrangement reactions crucial to cellular metabolism.
Cobalt is typically obtained as a by-product of nickel and copper refining. Processing routes involve flotation of sulfide ores, roasting/leaching, followed by solvent extraction/ion exchange to separate Co from Ni/Cu/Fe, and final recovery as Co metal or salts (e.g., CoSO4).
Co-60 is a gamma-emitting radioisotope used in radiotherapy, industrial radiography, and sterilization. It is produced by neutron activation of Co-59. Handling requires strict shielding and regulatory controls due to penetrating radiation.
Some cobalt compounds can be toxic or sensitizing (skin/respiratory). Dust and soluble salts should be minimized; use PPE and ventilation. Battery supply chains raise environmental and ethical sourcing concerns; recycling and responsible sourcing programs are increasingly emphasized.