I am a little confused when it comes to hydrated salts and transition metals.

When it comes to non transition metals like $\ce{Mg}$, we can clearly have $\ce{MgSO4.xH2O}$. However, when it comes to an element like $\ce{Fe}$, I do not know the difference between $\ce{Fe(OH)2}$ and $\ce{[Fe(OH)2 H2O4]}$, so I am unsure if we should write $\ce{[Fe(OH)2H2O4].xH2O}$ or $\ce{Fe(OH)2.xH2O}$.

  • $\begingroup$ "$\ce{CuSO4\cdot5H2O}$ can also be written as $\ce{[Cu(H2O)4]SO4\cdot H2O}$." This fact might be related to your question... $\endgroup$ Mar 16 at 11:19

1 Answer 1


$\ce{[Fe^{II}(OH)2(H2O)4]}$ is a neutral dissolved complex, formed from ferrous hydrated ion in alkalic solutions:

$$\ce{[Fe^{II}(H2O)6]^2+(aq) + 2 OH-(aq) -> [Fe^{II}(OH)2(H2O)4](aq) + 2 H2O(l)}$$

$\ce{Fe(OH)2}$ or $\ce{Fe(OH)2 . x H2O}$ is rather way to describe $\ce{Fe(OH)2(s)}$ that has rather undefined stoichiometric composition, being a result of condensation of the $\ce{[Fe^{II}(OH)2(H2O)4](aq)}$, releasing water and joining complexes with oxide or hydroxide bridging ligands.


$$\ce{2 [Fe^{II}(OH)2(H2O)4](aq) -> H2O(l) + [Fe^{II}(OH)(H2O)4-O-Fe^{II}(OH)(H2O)4](aq)}$$

$$\ce{2 [Fe^{II}(OH)2(H2O)4](aq) -> H2O(l) + [Fe^{II}(OH)(H2O)4-(OH)-Fe^{II}(OH)2(H2O)3](aq)}$$

and followed by further condensation to multicenter, polymeric substance of a typical amorphous look of transition metal hydroxides.

  • $\begingroup$ You said a lot of stuff that went over my head, but from what I can put together, the two iron compounds in my post are the same thing! $\endgroup$
    – nav
    Mar 16 at 14:39
  • $\begingroup$ If the stuff went over your head then yes, they are the same thing. :-) $\endgroup$
    – Poutnik
    Mar 16 at 14:58

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