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I am looking for reasons why $\ce{Mn(IV)}$ wouldn't be stable as a hydrate. I know that from standard reduction potentials it exists as a ppt of $\ce{MnO2}$, but what is stopping it from being a hydrated $\mathrm{4+}$ aqueous cation? Is it that the water ligands would be too polarised and therefore $\ce{H+}$ dissociation would occur very easily? Are there other factors that make it a solid rather than a solvated ion, like lattice enthalpy etc.?

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Solid $\ce{MnO2}$ can be hydrated, but it's the solid which precipitates from solutions as a product of redox reactions of other $\ce{Mn}$ compounds. There is also for example crystalline $\ce{[MnF4(py)(H2O)]}$ What about aquo cations you mentioned?

One thing is that $\ce{[Mn(H2O)_n]^4+}$ cations should be indeed quite strongly acidic, but that's no problem as another strong acid in solution would prevent precipitation (similar thing like with $\ce{Fe(OH)3}$). The problem is oxidative power, what may be surprising knowing that Mn has higher oxidation states.

Why do I think so? Well, let's see what happens if we try this trick with acid to make a solution. Concentrated $\ce{H2SO4}$ and $\ce{HCl}$ react with $\ce{MnO2}$, but lets see the reactions:

$$\ce{2 MnO2 + 2 H2SO4 → 2 MnSO4 + O2 + 2 H2O}$$ $$\ce{MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O}$$

In first case oxygen gets oxidised and in second chlorine!

Now what if we used $\ce{HF}$? Concentrated solutions are really powerful stuff! Unfortunately the problem persists, as you could see earlier $\ce{Mn^{4+}}$ can oxidise even water if acidity is high enough. With difficulty $\ce{MnF4}$ was synthesised, but guess what, Mn can gradually oxidise fluorine in this compound, making $\ce{F2}$ gas! That was first chemical reaction known to do that.

So, generally not much chance for simple aquo complexes.

Check out these articles on Wikipedia for more info and citations to articles: about $\ce{MnF4}$ and about $\ce{MnO2}$.

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  • $\begingroup$ >, but that's no problem as another strong acid in solution would prevent precipitation || It's a problem because really strong acid will dissociate in water anyway, a non-water solution would be needed. Monoatomic tetracations are a very rare thing a small cations like Mn definitely are not stable enough to exist in water solution. $\endgroup$ – permeakra May 24 '17 at 5:17
  • $\begingroup$ @permeakra Well concentrated H2SO4 is still arguably "aqueous" Yep, I was thinking about adding a lot of acid :D $\endgroup$ – Mithoron Jun 29 '17 at 23:46

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