manganese chemistry
Manganese is the showpiece of variable oxidation states. No other first-row transition metal spreads itself across so wide a range — from +2 right up to +7 — and each state has its own look and behavior. The deep purple of potassium permanganate, used to clean wounds, treat water and titrate in the lab, is the most familiar face, but the black solid in your spent batteries, the pink of a manganese(II) salt, and the brown stains on old stone are all manganese too.
The most stable everyday state is manganese(II), a d5 ion. Its pale pink color is extremely faint precisely because the d5 high-spin arrangement makes every possible d-d transition doubly forbidden, so it barely absorbs visible light. Manganese(IV) appears as the insoluble black-brown dioxide MnO2, the solid in dry-cell batteries and a useful oxidant. The crown is manganese(VII): the permanganate ion MnO4-, intensely purple not from d-d transitions (Mn7+ has no d electrons) but from a strong charge-transfer band, and a vigorous oxidizing agent in both acid and base. In strongly basic solution the green manganate(VI) ion MnO4 2- can form, and it famously disproportionates in acid, splitting into purple permanganate and brown MnO2 at once.
Manganese is woven through technology and life. It is essential in steelmaking (it mops up sulfur and oxygen and toughens the alloy), permanganate is a everyday oxidizer and disinfectant, and MnO2 has powered countless batteries. In biology a cluster of manganese atoms sits at the heart of photosystem II, the catalyst that splits water and releases the oxygen we breathe during photosynthesis. Manganese's wide range of stable and metastable states is exactly what makes it so useful as both an industrial reagent and a biological catalyst.
Manganate(VI), the green MnO4 2- ion, disproportionates in acid: 3 MnO4 2- + 4 H+ gives 2 MnO4- + MnO2 + 2 H2O. One manganese(VI) is oxidized up to purple permanganate(VII) while two are reduced down to brown manganese(IV) oxide — a single element splitting into a higher and a lower state at the same time.
Green manganate(VI) disproportionates in acid into purple permanganate(VII) and brown MnO2 (IV) at once.
Permanganate's intense purple is a charge-transfer band, not a d-d transition — manganese(VII) has zero d electrons, so it cannot make a d-d transition at all. The faint pink of manganese(II), by contrast, is a genuine but very weak d-d color, weak because the d5 transitions are spin-forbidden.