# Manganates (II)

I want to ask about character of manganese(II) compounds. I read that $\ce{MnO}$ is basic, just like $\ce{Mn(OH)2}$. I started doing research and I read ions like $\ce{MnO2^{2-}}$ and $\ce{HMnO2^-}$ exist too, but how? Are they related to reactions in solid state? Manganese(II) hydroxide isn't amphoteric like aluminium hydroxide so it's really confusing for me. I also found reaction that "confirms" amphoteric character of manganese(II) oxide

$$\ce{Mn(OH)2 + 2NaOH -> Na2[Mn(OH)4]}$$

Manganese(II) hydroxide is not only a strong base but is also amphoteric as it dissolves in strong alkali solution to form hydroxomanganates:

From the solutions of $\ce{Mn(OH)2}$ in 50% Sodium hydroxide solution, the hydroxometalates $\ce{Na2[Mn(OH)4)}]$ is isolated.

But before forming hydroxomangantes, intermediate ions like $\ce{MnO2^2-}$ and $\ce{HMnO2-}$ are formed. This paper(*) discuss this fact:

The high solubility of manganese hydroxide in acid solution and its low solubility in alkali solutions give a qualitative indication of the fact that manganese hydroxide is a strong base. This seems to be further verified by the observation that manganese salts do not hydrolyze appreciably. The complete verification of this prediction is given in the subsequent calculation of the acidic and basic constants of manganese hydroxide.

The following equations probably indicate the reaction that manganese hydroxide undergoes in water, in acid, and in alkali.

(...)

$$\ce{Mn(OH)2(s) + OH- -> HMnO2- + H2O }$$ $$\ce{Mn(OH)2(s) + 2OH- ->MnO2^2- + 2H2O}$$

(...)

The Equilibrium in Alkaline Solutions.

If manganese hydroxide is a strong base then we may expect its hydrogen ion dissociation to be very slight, particularly the second dissociation. Unfortunately, the solubility is so slight in the range where it is possible to calculate these acid dissociation values that no data were obtained.

(...)

$$\ce{Mn(OH)2 -> H+ + HMnO2- K = 1 \times 10^{-19}}$$

• No hydroxide can be amphoteric and also a strong base. If something dissociates hydroxide ions readily, why would it turn around and take in more? – Oscar Lanzi Apr 11 '17 at 21:11