cobalt and nickel chemistry

Cobalt and nickel are neighbors near the end of the first transition row, and together with iron they make up the trio of magnetic metals (all three are ferromagnetic). Cobalt gives us the deep blue of stained glass and old pottery glazes — 'cobalt blue' is named after it — and both metals are now in every headline about batteries, since lithium-ion cells lean heavily on cobalt and nickel oxides. By this point in the row the variable-oxidation-state spread has narrowed: both metals are dominated by the +2 state, with cobalt also reaching a stable +3 in complexes.

Cobalt(II), a d7 ion, is the workhorse, and it plays a famous color trick. The octahedral aqua ion [Co(H2O)6]2+ is pink, but the tetrahedral chloride complex [CoCl4]2- is intense blue, and the two interconvert as you change conditions — this is the basis of the cobalt chloride paper that turns from blue to pink as it picks up moisture. Cobalt(III) is unstable as a simple aqua ion (it oxidizes water), but in complexes with strong ligands like ammonia it becomes extremely stable and inert: the d6 low-spin [Co(NH3)6]3+ is the textbook example of a kinetically inert complex, which is exactly why Werner used cobalt(III) ammines to found coordination chemistry. Nickel sits almost entirely in nickel(II), a d8 ion, whose complexes show a lovely geometry choice: green octahedral [Ni(H2O)6]2+, blue octahedral [Ni(NH3)6]2+, and yellow square-planar complexes with strong ligands, where the d8 ion drops one orbital empty.

Both metals are industrially crucial. Nickel is the heart of stainless steels and superalloys and a famous hydrogenation catalyst (Raney nickel); cobalt is vital to high-strength alloys, magnets and lithium-ion battery cathodes. Cobalt is also the only one of these with a starring biological role: a cobalt(III) ion sits at the center of vitamin B12, one of the very few organometallic compounds nature uses. The lesson of cobalt and nickel is how, late in the row, the +2 state takes over and the interesting chemistry shifts into the geometry and stability of their complexes.