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Which is more stable in aqueous solution: $\ce{Cr^3+}$ or $\ce{Mn^3+}$? Why?

My approach: $\ce{Cr^{3+}}$ should be more stable as the $\ce{3d}$ electrons will enter the $\mathrm{t_{2g}}$ orbitals. Due to CFSE (Crystal Field Stabilization Energy) of half filled $\mathrm{t_{2g}}$ orbitals $\ce{Cr^{3+}}$ should be most stable.

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Both elements are stable under certain conditions in various oxidation states. In chromium they range from -2 to +6 and in manganese from 0 to +7.

However, some of them are more common than others, especially in aqueous solution. In chromium the most common are +3 and +6. As a "free" ion $\ce{Cr^{3+}}$ is most stable due to a filled s orbital and only one electron in the d shell. If you dissolve chromium(III)chloride in water you will get a complex formation. $$\ce{CrCl3 + 6H2O <=> [Cr(H2O)6]^{3+} + 3Cl-}$$ Assuming an octahedral ligand alignment in crystal field theory, you are correct in assuming, that the $t_{2g}$ orbitals will be half filled, resulting in a more stable configuration.

For manganese the most common oxidation states are +2, +4 and +7. If you dissolve manganese(III)chloride, then you will also form a aqueous complex, which will slowly transform to manganese(III)hydroxide and precipitate. $$\ce{MnCl3 + 6H2O <=> [Mn(H2O)6]^{3+} + 3Cl- <=> Mn(OH)3 v + 3H3+O + 3Cl-}$$ The resulting hydroxide may be oxidised by air to manganese(IV)oxide hydroxide. $$\ce{2\overset{\color{\green}{+III}}{Mn}(OH)3 + \overset{\color{\green}{\pm0}}{O2} -> 2\overset{\color{\green}{+IV}}{Mn}O(OH)2 + 2{}^{-}OH}$$ Over time this will convert into manganese dioxide. $$\ce{MnO(OH)2 -> MnO2 + H2O}$$

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  • $\begingroup$ Is it possible to generalize this stability of transition metals in aqueous solution for the ions $\ce{Co^+, Fe^3+, Cr^3+, Sc^2+}$ as well? Or should I post a new question? (on the precondition that you answer it as well ;) ) $\endgroup$ – Gaurang Tandon May 14 '18 at 17:51
  • $\begingroup$ @GaurangTandon Well, I think you'll be able to find stable "free" ions in aqueous solution for all of them, however, I'm not sure what you'd want to generalise here. I can give no guarantee that I would be able to write an answer to a follow-up question, that depends very much on my schedule and whether or not I can answer it quickly. However, this should not discourage you from seeking out the knowledge of the community. $\endgroup$ – Martin - マーチン May 14 '18 at 18:31
  • $\begingroup$ Okay! I was more hoping to compare their relative stability and generalize that trend to all 3d metal ions. Though I ended up posting a narrower question first instead: Compare stability of transition metal cations in aqueous solution $\endgroup$ – Gaurang Tandon May 15 '18 at 3:06
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Cr(III) is common as dirt and stable in solution as any number of different salts. Mn(III) is exotic, labile, and chemically reactive,

http://www.hach.com/asset-get.download.jsa?id=7639984472
DOI: 10.1039/AN9911600285
http://en.wikipedia.org/wiki/Manganese(III)_acetate
DOI:10.1039/C3RA42480D
interesting free radical one-electron oxidation reagent

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