Manganese (Mn)

Manganese has the ground-state configuration \([Ar]3d^5\,4s^2\). The availability of both 3d and 4s electrons for bonding and the stability of half-filled \(3d^5\) favor multiple oxidation states from +2 to +7, giving rich redox behavior.

• +2 (Mn²⁺): pale pink, often \([\mathrm{Mn(H_2O)_6}]^{2+}\)
• +4 (MnO₂): brown/black solid
• +6 (manganate, \(\mathrm{MnO_4^{2-}}\)): green
• +7 (permanganate, \(\mathrm{MnO_4^-}}\)): deep purple

Intermediate states (+3, +5) also exist but are less common in water.

In \(\mathrm{MnO_4^-}\) manganese is in a very high oxidation state (+7) and readily gains electrons. The reduction products depend on pH:

• Acidic: \(\mathrm{MnO_4^- + 8\,H^+ + 5\,e^- \rightarrow Mn^{2+} + 4\,H_2O}\)
• Neutral: \(\mathrm{MnO_4^- + 2\,H_2O + 3\,e^- \rightarrow MnO_2(s) + 4\,OH^-}\)
• Basic: \(\mathrm{MnO_4^- + e^- \rightarrow MnO_4^{2-}}\)

Natural MnO₂ (pyrolusite) is fused with KOH and an oxidant (air/Cl₂) to form green potassium manganate:

\(\mathrm{2\,MnO_2 + 4\,KOH + O_2 \rightarrow 2\,K_2MnO_4 + 2\,H_2O}\)

Then disproportionation or electrolytic oxidation converts \(\mathrm{K_2MnO_4}\) to purple \(\mathrm{KMnO_4}\):

\(\mathrm{3\,K_2MnO_4 \rightarrow 2\,KMnO_4 + MnO_2\downarrow + 2\,KOH}\)

Acidic permanganate (">self-indicating") oxidizes many reductants, with the endpoint seen as a persistent faint pink. A classic reaction is with Fe²⁺:

\(\mathrm{MnO_4^- + 5\,Fe^{2+} + 8\,H^+ \rightarrow Mn^{2+} + 5\,Fe^{3+} + 4\,H_2O}\)

Mn acts as a deoxidizer and sulfur fixer, forming MnS inclusions that reduce hot-shortness, and it solid-solution strengthens steel. Ferroalloy additions (e.g., ferromanganese) are standard in producing structural and stainless steels.

In alkaline Zn–MnO₂ cells, MnO₂ serves as the cathode depolarizer, being reduced while Zn is oxidized. A simplified cathodic step:

\(\mathrm{2\,MnO_2 + H_2O + 2\,e^- \rightarrow Mn_2O_3 + 2\,OH^-}\)

More detailed mechanisms involve formation of MnOOH intermediates.

Green manganate \(\mathrm{MnO_4^{2-}}\) disproportionates in neutral/acidic solution to brown MnO₂ and purple permanganate:

\(\mathrm{3\,MnO_4^{2-} + 4\,H^+ \rightarrow 2\,MnO_4^- + MnO_2\downarrow + 2\,H_2O}\)

Strongly alkaline conditions suppress this, stabilizing manganate.

Mn is a trace essential element—cofactor in enzymes like Mn-superoxide dismutase and involved in photosystem II water splitting (Mn₄CaO₅ cluster). However, excessive exposure (dust/fumes) can be neurotoxic. Appropriate industrial hygiene (PPE, ventilation) is necessary.

Mn²⁺ can be oxidized by strong oxidants (e.g., sodium bismuthate) in acidic medium to purple permanganate for spectrophotometric detection:

\(\mathrm{2\,Mn^{2+} + 5\,BiO_3^- + 14\,H^+ \rightarrow 2\,MnO_4^- + 5\,Bi^{3+} + 7\,H_2O}\)

This high-valent product provides a sensitive colorimetric handle.